WO1992006510A1

WO1992006510A1 - Multilayer ceramic structures

Info

Publication number: WO1992006510A1
Application number: PCT/GB1991/001675
Authority: WO
Inventors: Steven John Butcher; James David Hobby
Original assignee: Cookson Group Plc
Priority date: 1990-09-28
Filing date: 1991-09-27
Publication date: 1992-04-16
Also published as: GB9021121D0

Abstract

A multilayer ceramic actuator or transducer which comprises a stack formed from a plurality of layers of a dielectric material having a plurality of internal electrodes sandwiched therebetween, the stack having two tabs or protrusions formed thereon and alternate internal electrodes extending only into one or other of the tabs or protrusions, the internal electrodes formed in the tabs or protrusions being connected to conductive terminations, characterized in that for each electrode layer the surface area of the unelectroded tab portion is less than 5 % of the total surface area of the electroded portion.

Description

MULTILAYER CERAMIC STRUCTURES

The present invention relates to multilayer ceramic structures and, in particular, to multilayer ceramic actuators or transducers in which a plurality of dielectric layers are separated by conducting electrodes and a method for the fabrication thereof.

Multilayer ceramic structures in which a plurality of dielectric layers are separated by conducting electrodes are well known in the art. In such structures, the alternate electrode layers are connected to one another. However, the electrical connections to the alternate electrode layers are difficult to make because the inter-electrode distance is small. In the prior art various methods are disclosed for preventing contact between adjacent electrodes.

Conventionally, in order to facilitate the connection of electrodes of the same polarity without shorting between the electrodes of adjacent layers, a stack of green ceramic sheets is formed with the electrodes of opposing polarity being exposed at different surfaces of the stack. In order to form such an arrangement, the sheets forming the stack are coated with an electrode material in a manner such that the electrode area extends to one margin of the stack but terminates short of the opposite margin of the stack. It is thus possible to build up a stack in which the electrodes of one polarity are exposed at one surface of the stack and the electrodes of the opposite polarity are exposed at the opposite surface of the stack. The adjacent electrodes are isolated one from the other by the unelectroded ceramic areas intervening between them. When such structures are used as ceramic actuators there has been found to be a tendency for them to split or crack after prolonged use. This is believed to be due to the layer to layer ceramic bonding in the unelectroded areas constraining the ability of the structure to expand in a direction perpendicular to the plane of the layers of dielectric material.

U.S. Patent No. 4,523,121 describes a multilayer electrostrictive device capable of enhanced expansion characteristics and of withstanding repeated pulsing. This reference notes the constraining influence of bridging areas of ceramic between dielectric layers and it proposes, as a solution, the formation of an essentially conventional electrostrictive device which is modified by recessing or cutting away the bridging ceramic areas between adjacent layers.

U.S. Patent No. 4,681,667 describes a method of manufacturing an electrostrictive device by forming a monolith of alternate ceramic and electrode layers. Electrodes of a first polarity are electrically connected and the device is immersed in a bath containing charged glass insulating particles which are attracted to the exposed portions of the electrodes remote from the initial connection. The process is repeated with the electrodes of opposite polarity, thereby forming surfaces on which electrodes of a given polarity only are exposed. Connections may thereafter be effected to the surfaces without fear of the connecting material shorting electrodes of opposite polarities. U.S. Patent No. 4,805,057 describes a multilayer ceramic structure which comprises a piezoelectric ceramic sintered body formed from a plurality of electrodes stacked alternately with ceramic layers. The plurality of electrodes includes first and second electrode groups which are electrically connected to each other by conductors in through holes, the through holes being provided in portions of the sintered body where unelectroded areas are already superimposed over electroded areas.

JP-1-300577 discloses a multilayer ceramic structure in which the faces of the ceramic layers forming the multilayer structure are etched so that the end faces of the internal electrodes project alternately from different faces of the multilayer structure. IBM Technical Disclosure Bulletin, Vol.25, No. 10, March 1983, discloses multilayer ceramic capacitor chips in which the thermal expansion mismatch between the multilayer ceramic capacitor chip and the substrate is reduced. The reduction is accomplished by stacking the capacitor sheets orthogonally to the line between the attachment pads instead of stacking as in the traditional packages. A notch is provided in each layer of the stack which separates the two electrodes of the capacitor and with this arrangement there is a reduction of the distance between the centre of the capacitor and the negative and positive solder pads. The most favourable architecture for the capacitor is thus one where the gap formed by the notches in the layers of the stack is as small as possible with the relatively large tabs extending across the majority of the capacitor. However, the use of this arrangement as an actuator or transducer, because of the relatively large area of the tabs, would lead to differential strains, as in the conventional arrangements. We have now surprisingly found that it is possible to reduce the differential strains in multilayer ceramic actuator or capacitor structures by making the electrode connections to relatively small tabs or protrusions formed on the device.

Accordingly, in one aspect the present invention provides a method for the fabrication of a multilayer ceramic actuator or transducer in which a plurality of layers of a dielectric material are separated by internal electrodes, which method comprises the steps of: a) preparing a laminated structure having alternating layers of a dielectric t material and of internal electrodes, the layers of the dielectric material being formed with two tabs or protrusions thereon and alternate layers of the internal electrodes being superposed over the dielectric material except in the region of one of the tabs or protrusions, with the alternate layers of the internal electrodes being superposed over different of the tabs or protrusions, b) heating and sintering the laminated structure, and c) applying conductive terminations to the internal electrodes formed in the tabs or protrusions of the laminated structure , characterized in that for each electrode layer the surface area of the unelectroded tab portion is less than 5% of the total surface area of the electroded portion. In another aspect the present invention provides a multilayer ceramic actuator or transducer which comprises a stack formed from a plurality of layers of a dielectric material having a plurality of internal electrodes sandwiched therebetween, the stack having two tabs or protrusions formed thereon and alternate internal electrodes extending only into one or other of the tabs or protrusions, the internal electrodes formed in the tabs or protrusions being connected to conductive terminations, characterized in that for each electrode layer the surface area of the unelectroded tab portion is less than 5% of the total surface area of the electroded portion.

It is an essential feature of the actuators and transducers of the present invention that the tabs or protrusions are small compared to the area of the device and the architecture of the devices of the present invention is thus not suitable for surface mounted capacitors. As previously specified, for each layer in the device the area of the unelectroded tab portion is less than 5% of the total surface area of the electroded portion. It is furthermore preferred that the tab area should be as small as possible, although there are some practical constraints. Theoretically, the minimum tab protrusion is determined by the electrical breakdown between the face of the tab and the actuator body, as defined below: V = E_Ld_L where V is the applied voltage.

EL is the field on the actuator, and dj_j is the inter electrode separation. The field, E_τ, between the tab and the body of the device is

E_τ = V/Th < E (breakdown) where T^ is the height or protrusion of the tab. Therefore, T_h > V/E_T = E_LdL/E_T

In general, therefore, T^ should be > d_L. The minimum width of the tab is determined by the methods of forming the conductive terminations. For example, a screen printed conductive track of the order of 100 micrometres can readily be formed. In carrying out the method of the present invention any number of sheets of a dielectric material may be formed into the laminate, depending upon the intended end use of the structure. Typically 50 to 100 layers may be used. Furthermore, it is possible to form a stack of two or more mutlilayer ceramic actuators or transducers in accordance with the invention. Thus, a plurality of .-stacks may be joined together, for example by bonding with an epoxy resin, to form a unit comprising a large number of layers. For example, joining together four stacks each having 100 layers would provide a unit having 400 layers. It will be appreciated by those skilled in the art that it is easier to manufacture four stacks of 100 layers each, than to manufacture a single stack having 400 layers.

The conductive terminations are preferably formed by the steps of d) connecting the internal electrodes formed in one of the tabs or protrusions of the laminated structure to an external electrode of one polarity, and e) connecting the internal electrodes formed in the other of the tabs or protrusions of the laminated structure to an external electrode of the opposite polarity. The multilayer ceramic actuator or transducer so formed, in which one set of alternate internal electrodes formed in one tab or protrusion is connected to an external electrode of one polarity and the other set of alternate internal electrodes formed in the other tab or protrusion is connected to an external electrode of the opposite polarity, is preferably an actuator. The dielectric material may be a piezoelectric or electrostrictive material when the structure is intended for use as an actuator. Generally, green ceramic sheets are coated in the required area with an electrically conductive ink or paste which forms the internal electrode layers. The ink or paste is alternately omitted from one tab or protrusion of each c ramic layer so that an area which is electroded on one ceramic sheet is above and/or below an area or another ceramic sheet which is not electroded. The electrically conductive ink or paste is preferably screen printed onto the sheets of the ceramic material. The electrically conductive ink or sheet may include therein any suitable dielectric material.

The laminated structure which is prepared in the above manner is then subjected to heating and sintering to provide a sintered body.

The procedure described above for the formation of the multilayer ceramic actuators or transducers of the present invention may be modified for the production of a plurality of devices. In this case, larger sheets of the green ceramic material are taken and, coated in the required areas with a electrically conductive ink or paste in the manner as described above. A plurality of the coated sheets are then laminated together and the individual structures cut out, for example with a laser cutting device or a diamond tool, before or after heating and sintering. A plurality of sintered bodies is thus obtained.

The internal electrodes formed in the tabs or protrusions of the laminated structure are connected to electrodes of opposite polarity, preferably by depositing conducting tracks onto an exposed face of the respective tabs or protrusions.

Preferably, the tabs or protrusions are formed on the same side of the laminated structure, with a laminated structure of an essentially U-shaped configuration being particularly preferred.

The present invention will be further described with reference to the following drawings in which:

Figure 1 is a cross-sectional diagrammatic view of an actuator;

Fi'gure 2 is a plan view of the electroded layers of a structure in accordance with the invention; Figure 3 is a perspective view of an actuator in accordance with the invention; and

Figure 4 is a strain-field curve for a device in accordance with the invention.

Referring to the drawings, Figure 1 illustrates a structure in which layers 1 of a dielectric material are separated by electrode layers 2. Alternate electrode layers are connected together on one side of the schematic structure to a positive potential as shown at 3 whilst the alternate sets of electrodes are connected at the otherside of the schematically shown structure to a negative potential as shown at 4.

With reference to Figures 2 and 3 layers 5 of an essentially U-shaped configuration are formed. As best shown in Figure 2, the layers each have 2 tabs, 6 and 7. As shown in Figure 2A the ceramic layer is electroded over all of the surface thereof, except for tab 7, whilst in Figure 2B the reverse in the case with the ceramic layer being electroded over the surface thereof except for tab 6. The layers A and B are arranged alternately to form a multilayer structure as shown in Figure 3. The exposed electrodes are shown at 8. In tab 6 of the multilayer structure alternate electrodes are connected by conducting track 9 which is connected to a positive potential at 10. The other set of alternate electrodes in tab 7 is connected to a conducting track 11 to a negative potential at 12.

The strain-field data was measured for a multilayer ceramic actuator as shown generally in Figure 2. The device comprised a total of 20 layers, the thickness of each dielectric layer being 125 micrometres and the thickness of each electrode layer being 10 micrometres. The active area of each electrode layer in the device was 90mm² and the inactive (tab) area of the layer 3mm². The results are given in Figure 4.

The operational lifetime of an example of this type of device was very high and there was no degradation is behaviour after 20 X lθ⁹ input voltage cycles, corresponding to more than 40 billion movements.

Claims

CLAIMS :

1. A method for the fabrication of a multilayer ceramic actuator or transducer in which a plurality of layers of a dielectric material are separated by internal electrodes, which method comprises the steps of:

» a) preparing a laminated structure having alternating layers of a dielectric material and of internal electrodes, the layers of the dielectric material being formed with two tabs or protrusions thereon and alternate layers of the internal electrodes being superposed over the dielectric material except in the region of one of the tabs or protrusions, with the alternate layers of the internal electrodes being superposed over different of the tabs or protrusions, b) heating and sintering the laminated structure, and c) applying conductive terminations to the internal electrodes formed in the tabs or protrusions of the laminated structure, characterized in that for each electrode layer the surface area of the unelectroded tab portion is less than 5% of the total surface area of the electroded portion.

2. A method as claimed in claim 1 wherein the conductive terminations are formed by: d) connecting the internal electrodes formed in one of the tabs or protrusions of the laminated structure to an external electrode of one polarity, and e) connecting the internal electrodes formed in the other of the tabs or protrusions of the laminated structure to an external electrode of the opposite polarity. t

3. A method as claimed in claim 2 wherein the external electrode connections are formed by depositing conducting tracks onto an exposed face of the respective tabs or protrusions.

4. A method as claimed in any one of the preceding claims wherein the tabs or protrusions are formed on the same side of the laminated structure.

5. A method as claimed in any one of the preceding claims wherein the laminated structure is of essentially U-shaped configuration.

6. A method as claimed in any one of the preceding claims wherein the internal electrode layers are formed from an electrically conductive ink or paste.

7. A multilayer ceramic actuator or transducer which comprises a stack formed from a plurality of layers of a dielectric material having a plurality of internal electrodes sandwiched therebetween, the stack having two tabs or protrusions formed thereon and alternate internal electrodes extending only into one or other of the tabs or protrusions, the internal electrodes formed in the tabs or protrusions being connected to conductive terminations, characterized in that for each electrode layer the surface area of the unelectroded tab portion is less than 5% of the total surface area of the electroded portion.

8. A multilayer ceramic structure as claimed in claim 7 wherein one set of alternate internal electrodes formed in one tab or protrusion is connected to a external electrode of one polarity and the othter set of alternate internal electrodes formed in the other tab or protrusion is connected to an external electrode of the opposite polarity.

9. A multilayer ceramic structure as claimed in claim 7 or claim 8 wherein the tabs or protrusions are formed on the same side of the laminated structure.

10. A multilayer ceramic structure as claimed in any one of claims 1' to 9 wherein the laminated structure is of essentially U-shaped configuration.