PIEZO-MULTILAYER ACTUATOR WITH IMPROVED ELECTRODE CONTACT

Abstract: A piezo-multilayer actuator having a plurality of piezoceramic layers (2) arranged in a stack and a plurality of inner electrodes (22, 32) interleaved in the piezoceramic layers (2), the plurality of inner electrodes extending from first and second sides of the stack in alternating fashion. First and second outer electrodes (42, 52) are connected to the plurality of inner electrodes extending respectively from the first and second sides of the stack. The inner electrodes include thickened portions (26) at the first and second connection areas, the thickened portions being thicker than other portions of the plurality of inner electrodes so as to reinforce the inner electrodes. Cracks (72) which may form on the inner electrodes due to tensile stresses tend to migrate around the thicker portions of the inner electrodes at the first and second connection areas. Propagation of the cracks to the outer electrodes with the attendant possibility of current interruption in the outer electrodes, is thereby reduced or halted.

Description: PIEZO-MULTILAYER ACTUATOR WITH IMPROVED ELECTRODE CONTACT Field of the Invention The present invention relates generally to piezo-multilayer devices, and, in particular, to a piezoceramic multilayer actuator with a crack stopper for a diesel injection device. Background Information Actuators employing multilayer piezoceramic devices may be used in diesel fuel injection devices for internal combustion engines. Piezoceramics have the property that under mechanical pressure or tension they develop a charge, and when charged electrically, they expand or contract. To intensify this effect, monolithic multilayer actuators are used which are composed of a sintered stack of thin foils of piezoceramic, e.g., lead zirconate titanate with imbedded metallic inner electrodes. Inner electrodes extend in alternating fashion out of the stack and are connected in parallel via outer electrodes. On the contact sides of the stack, a primary metallic layer, or outer electrode, is applied which is connected to the individual inner electrodes. If an electric voltage is applied to the outer electrodes, the piezofoils expand in the field direction. Through mechanically connecting the individual piezofoils in series, the nominal expansion of the entire piezoceramic is achieved even at low electrical voltages. These monolithic multilayered actuators are described in detail in German Patent No. 40 36 287 C2. Application of such actuators in a flow regulating valve is also described therein. The inner electrodes of the piezo-multilayer actuator may be screen printed from approximately 4-μm-thick Ag paste or, in rare cases, sputtered from approximately 1 -μm-thick CuNi layer. As a result of the inner electrodes being alternately offset back from the outer electrodes for electrical insulation purposes, zones with weakened electrical fields develop at the outer edges of the piezofoil stack so that in these areas the piezofoils expand less than in the central area of the stack. As a result, tensile stresses occur in the outside zones and after the adhesive strength is exceeded, delamination develops between the piezofoil layers and the inner electrodes and cracks form on the electrodes. The cracks may migrate outward into the outer electrodes and lead to interruption of current if the crack goes all the way through. German Patent Application No. 1 96 48 545 A1 describes a method of improving a monolithic multilayer actuator to avoid destruction of the multilayer actuator even at high dynamic loads. The application describes a three-dimensionally textured, electrically-conductive side electrode which is connected via soldering at discrete contact points with the outer electrode and is configured to be expandable between the contact points. For example, a wave-shaped side electrode with troughs at the contact points is described. The operating current of the actuator is split through this arrangement into secondary currents. Secondary currents flow from the contact points through the outer electrodes to the metallic inner electrodes. Cracks which occur in the outer electrode lead only to a rerouting of the secondary currents through the three-dimensional electrode, without current being interrupted. However, the side electrode is an additional component which must be soldered into place, entailing additional cost. Also, shear cracks between the solder and the side electrode can result in detachment of the side electrode. Summary of the Invention The present invention provides a piezo-multilayer actuator including a plurality of layers of piezoelectric material arranged in a stack having first and second sides, a plurality of inner electrodes arranged between the plurality of layers of piezoelectric material, the plurality of inner electrodes extending from the first and second sides in alternating fashion so as to form respectively a first set of inner electrodes and a second set of inner electrodes. A first outer electrode is disposed on the first side of the stack, the first outer electrode being connected at first connection areas to the first set of inner electrodes, and a second outer electrode is disposed on the second side of the stack, the second outer electrode being connected at second connection areas to the second set of inner electrodes. The plurality of inner electrodes include thickened portions at the first and second connection areas, the thickened portions being thicker than other portions of the plurality of inner electrodes. The present invention also provides a method of manufacturing a piezo- multilayer actuator, the method including providing a plurality of layers of piezoelectric material arranged in a stack having first and second sides, and providing a plurality of inner electrodes arranged between the plurality of layers of piezoelectric material, the plurality of inner electrodes extending from the first and second sides in alternating fashion so as to form respectively a first set of inner electrodes and a second set of inner electrodes. The method further includes providing a first outer electrode disposed on the first side of the stack, the first outer electrode being connected at first connection areas to the first set of inner electrodes, providing a second outer electrode disposed on the second side of the stack, the second outer electrode being connected at second connection areas to the second set of inner electrodes, and thickening the plurality of inner electrodes at the first and second connection areas so as to form thickened portions, the thickened portions being thicker than other portions of the plurality of inner electrodes. Brief Description of the Drawings Fig. 1 shows a schematic cross-sectional view of a prior piezo-multilayer actuator depicting the electric field lines. Fig. 2 shows a detail of the prior piezo-multilayer actuator of Fig. 1 depicting delamination and cracking of electrodes. Fig. 3 shows a schematic cross-sectional view of a portion of a piezo-multilayer actuator according to the present invention. Fig. 4 shows a detail of the actuator of Fig. 3 depicting the redirection of cracks. Fig. 5 shows a schematic cross-sectional view a portion of a piezo-multilayer actuator according to the present invention having a wave-shaped outer electrode. Fig. 6 shows a schematic cross-sectional view of a portion of a piezo-multilayer actuator according to the present invention with an outer electrode having elongated contact areas and areas extending away from the stack between contact areas. Detailed Description Fig. 1 shows a prior piezo-multilayer actuator 1 0 with the electric field lines indicated by the letter "E. " The actuator 1 0 includes stack 1 2 of thin piezoceramic foils 2. First and second sets of inner electrodes 22 and 32, respectively, are interleaved in foils 2 and extend in alternating fashion out of stack 1 2 on opposite sides 1 4 and 1 6, respectively. First inner electrodes 22 are each connected to first outer electrode 42 at a respective connection point 24, while second inner electrodes 32 are each connected to second outer electrode 52 at a respective connection point 34. First inner electrodes 22 are offset from second outer electrode 52 and second inner electrodes 32 are offset from first outer electrode 42, as shown, to provide electrical insulation between first inner electrodes 22 and second outer electrode 52, and between second inner electrodes 32 and first outer electrode 42, respectively. In areas 62 formed by these offsets, the electric field formed by first inner electrodes 22 and second inner electrodes 32 is distorted, as indicated by the letter "E . " The electrical field is consequently weakened in areas 62. Referring now to Fig. 2, in areas 62 the piezofoils expand less than in other areas of stack 1 2, causing stresses. The result may be delamination 70 and cracks 72. Cracks 72 may migrate outward and finally form discontinuities 74 in first outer electrode 42 or second outer electrode 52, interrupting current flow in the respective outer electrode. Fig. 3 depicts a portion of an embodiment of a piezo-multilayer actuator 1 0 according to the present invention. The interleaved, offset arrangement of first inner electrodes 22 and second inner electrodes 32, in stack 1 2 of piezoceramic foils 2, as well as the arrangement of and connections of first outer electrode 42 and second outer electrode 52 is substantially similar to the prior actuator described above. As embodied herein, however, first inner electrodes 22 and second inner electrodes 32 are provided with thickened portions 26 in the area of connection points 24 and 34, respectively, where the inner electrodes are connected to first and second outer electrodes 42 and 52, respectively. Thickened portions 26 are thicker than other areas of the inner electrodes, thus reinforcing the inner electrodes at connection points 24 and 34. Referring now to Fig. 4, with implementation of thickened portions 26 according to the present invention, delaminations 70 and cracks 72 may develop, but the cracks tend to migrate around thickened portions 26 and to be confined to smaller areas. This redirecting of the cracks consumes cracking energy, shortening the net propagation length of the crack. Thickened portions 26 increase the effective expansion length of cracks 72 in the expansion direction "y" (Fig. 4) in outer electrodes 42 and 52. The tension within the outer electrode due to the crack is thereby reduced. Thickness "X" (Fig. 4) of the thickened portions may be selected so that the tension in the outer electrode is reduced below the tensile strength of the electrode, preventing the crack from completely penetrating the outer electrode. Further beneficial effects may be achieved by selecting tensile and adhesive strengths of outer electrodes 42 and 52 at thickened portions 26 so that the tensile strength is greater than the adhesion strength. This will tend to cause a crack to follow the profile of a thickened portions without penetrating the entire outer electrode. Thus, thickened portions 26 reduce the tendency of cracks 72 to propagate, and may stop the cracks entirely. Thickened portions 26 are preferably generally spheroid in shape, but in other embodiments can be any of a variety of shapes. Other suitable shapes include hemispherical or mushroom-shaped and elliptical shapes. In general, a curved profile without excessive corners is suitable. Thickened portions 26 may be made using a variety of, or combinations of, processes. Electroplating is a suitable process for making the thickened portions. Other suitable processes include photomasking techniques, chemical deposition processes, silk screen printing techniques and photomask with silk screen printing. Edges of the thickened portions may be rounded in the forming process itself, by using special photomask shapes or through a sintering process in silk screen printing. Thickened portions 26 are preferably made of electrically conductive materials, such as metals including copper, nickel, silver, gold, etc., or metal alloys including silver-palladium, brass, bronze, etc. In other embodiments of the actuator according to the present invention, thickened portions 26 may be multiple layers of metals or metal alloys for improved adhesion or reduced corrosion (for example, using a gold coating), or to adapt the thermal expansion properties (for example, to adapt to an inner electrode 42 or 52 made of a silver- palladium alloy). Referring to Fig. 5, first and second outer electrodes 42 and 52, respectively, may be wave-shaped, as with the side electrodes described in German Patent Application No. 1 96 48 54 A1 , discussed above. Wave-shaped first and second outer electrodes 42 and 52, respectively, may be connected-via soldering or welding, for example-directly to the thickened portions 26 of first and second inner electrodes 22 and 32, respectively, according to the present invention. Wave-shaped outer electrodes may avoid cracks in the outer electrodes altogether. Cracks tend to travel along the interface 78 between piezoceramic foils 2 and the metal of an inner electrode 22 or 32. When a crack 72 reaches side 1 4 at an ceramic-thickened portion-air junction 80, as shown in Fig. 5, the crack will tend to travel around the thickened portion, rather than into it. Additionally, wave-shaped outer electrodes are flexible in the expansion direction "y" of piezoceramic foils 2, minimizing tensile and shearing stresses at connection points caused by expansion of the piezoceramic actuator. Thickened portions 26 also simplify such direct connection of a wave-shaped outer electrode by enlarging the effective soldering area of an inner electrode 22 or 32. The inner electrodes are often quite thin (e.g., 4 μm) , making, in the absence of the thickened portions, such direct connection difficult and producing unreliable electrical connections. For proper connection to thickened portions 26, the wave length of wave-shaped first and second outer electrodes 42 and 52, respectively, is preferably equal to twice the spacing between the piezofoils, i.e., equal to the spacing of first inner electrodes 22 and the spacing of second inner electrodes 32. Referring now to Fig. 6, in another embodiment of an actuator according to the present invention, first and second outer electrodes 42 and 52, respectively, are provided with alternating substantially straight sections 44 and curved sections 46 which extend away from stack 1 2. In this embodiment of the present invention, straight sections 44 are connected directly to thickened portions 26 of first and second inner electrodes 22 and 32, respectively. Advantages of the wave-shaped electrode described above are retained. Additionally, straight sections 44 permit easier connection of first and second outer electrodes 42 and 52, respectively, to thickened portions 26 by providing elongated connection surfaces 45, proper connection being less dependent on the spacing between first and second inner electrodes 22 and 32, respectively.

Claims: WHAT IS CLAIMED IS: 1 . A piezo-multilayer actuator comprising: a plurality of layers of piezoelectric material arranged in a stack having first and second sides; a plurality of inner electrodes arranged between the plurality of layers of piezoelectric material, the plurality of inner electrodes extending from the first and second sides in alternating fashion so as to form a first set of inner electrodes and a second set of inner electrodes, respectively; a first outer electrode disposed on the first side of the stack, the first outer electrode being connected at first connection areas to the first set of inner electrodes; and a second outer electrode disposed on the second side of the stack, the second outer electrode being connected at second connection areas to the second set of inner electrodes; wherein the plurality of inner electrodes include thickened portions at the first and second connection areas, the thickened portions being thicker than other portions of the plurality of inner electrodes. 2. The piezo-multilayer actuator according to claim 1 wherein the plurality of layers of piezoelectric material include piezoceramic layers. 3. The piezo-multilayer actuator according to claim 1 wherein the thickened portions are formed using at least one of electroplating, photomasking, chemical deposition and silk screening. 4. The piezo-multilayer actuator according to claim 1 wherein the thickened portions include at least one of copper, nickel, silver, gold and a metal alloy. 5. The piezo-multilayer actuator according to claim 4 wherein the metal alloy includes at least one of silver-palladium, brass and bronze. 6. The piezo-multilayer actuator according to claim 1 wherein the thickened portions are formed in multiple layers of at least one of at least one metal and at least one metal alloy. 7. The piezo-multilayer actuator according to claim 1 wherein the thickened portions are capable of reducing propagation of stress cracks. 8. The piezo-multilayer actuator according to claim 1 wherein the thickened portions are have a shape including at least one of spheroidal, hemispherical, mushroom-shaped and elliptical. 9. The piezo-multilayer actuator according to claim 1 wherein at least one of the first and second outer electrodes extends away from the stack between the respective first and second connection areas. 1 0. The piezo-multilayer actuator according to claim 9 wherein the at least one of the first and second outer electrodes includes substantially straight portions at the respective first and second connection areas. 1 1 . The piezo-multilayer actuator according to claim 9 wherein the at ieast one of the first and second outer electrodes is wave-shaped. 1 2. The piezo-multilayer actuator according to claim 1 wherein the actuator is included in a diesel injection device. 1 3. A method of manufacturing a piezo-multilayer actuator, the method comprising: providing a plurality of layers of piezoelectric material arranged in a stack having first and second sides; providing a plurality of inner electrodes arranged between the plu rality of layers of piezoelectric material, the plurality of inner electrodes extending from the first and second sides in alternating fashion so as to form respectively a first set of inner electrodes and a second set of inner electrodes; providing a first outer electrode disposed on the first side of the stack, the first outer electrode being connected at first connection areas to the first set of inner electrodes; providing a second outer electrode disposed on the second side of the stack, the second outer electrode being connected at second connection areas to the second set of inner electrodes; and thickening the plurality of inner electrodes at the first and second connection areas so as to form thickened portions, the thickened portions being thicker than other portions of the plurality of inner electrodes. 14. The method according to claim 1 3 wherein the plurality of layers of piezoelectric material include piezoceramic layers. 1 5. The method according to claim 1 3 wherein the thickening of the inner electrodes is performed using at least one of electroplating, photomasking, chemical deposition and silk screening. 1 6. The method according to claim 1 3 wherein the thickened portions include at least one of copper, nickel, silver, gold and a metal alloy. 1 7. The method according to claim 1 6 wherein the metal alloy includes at least one of silver-palladium, brass and bronze. 1 8. The method according to claim 1 3 wherein the thickened portions are formed in multiple layers of at least one of at least one metal and at least one metal alloy. 1 9. The method according to claim 1 3 wherein the thickening of the inner electrodes is performed at the first and second connection areas so as to reduce propagation of stress cracks. 20. The method according to claim 1 3 wherein the thickened portions have a shape including at least one of spheroidal, hemispherical, mushroom-shaped and elliptical. 21 . The method according to claim 1 3 wherein at least one of the first and second outer electrodes extends away from the stack between the respective first and second connection areas. 22. The method according to claim 21 wherein the at least one of the first and second outer electrodes includes substantially straight portions at the respective first and second connection areas. 23. The method according to claim 21 wherein the at least one of the first and second outer electrodes is wave-shaped. 24. The method according to claim 1 3 wherein the actuator is included in a diesel injection device.

Inventors: HEINZ RUDOLF, HEINZ, RUDOLF
Applicants: BOSCH GMBH ROBERT, ROBERT BOSCH GMBH
Application year: 2000
Main classification: H01L41
IPC codes: F02M51, H01L41