Piezoelectric multilayer component

A method for producing a piezoelectric multilayer component, as well as a piezoelectric multilayer component with a range of reduced mechanical stability which can be produced by the method, is specified.

Out <patcit id="pcit0001" dnum="DE102006031085A1"><text>DE 10 2006 031 085 A1</text></patcit> A piezoelectric multilayer component with predetermined breaking layers is known. The<patcit id="pcit0002" dnum="WO2007097460A"><text>WO 2007/097460 A</text></patcit> Discloses a piezoelectric multilayer device which includes electrode layers and clotted layers.

A problem to be solved is to specify a piezoelectric multilayer component which can be operated stably over a period as long as possible.

A piezoelectric multilayer component is provided as an intermediate product with a stack of green piezoceramic layers arranged one above the other and alternating superposed electrode layers according to claim 1.

A first electrode layer is deposited on a piezoceramic layer and contains a first metal. A sacrificial layer is applied to a further piezoceramic layer and adjacent to the first electrode layer in the stacking direction, the sacrificial layer containing the first metal in a higher concentration than the first electrode layer. The term "concentration" means the weight fraction of the metal in the respective electrode layer.

If the intermediate product is sintered, the first metal diffuses from the sacrificial layer to the first electrode layer, leaving cavities thereby forming a desired breaking layer. The sacrificial layer is thus a metallic layer which, by virtue of its material loss during sintering or even because of its structuring during operation, can not take up any function as an electrode layer and is therefore also referred to as a "sacrificial" layer. Preferably, the ratio between the concentration of the first metal in the sacrificial layer and the first metal in the first electrode layer is selected in such a way that the sacrificial layer loses sufficient material in order, in particular, to not be able to have any electrical contact with any outer contacts present on the outer surfaces of the stack.

The first electrode layer and the sacrificial layer form a paired system in which a material exchange takes place upon heating of the piezoelectric multilayer component.

Preferably, the sacrificial layer is designed as a structured sacrificial layer. In this case, it has a discontinuous structure, ie it covers only a part of the green piezoceramic layer. The sacrificial layer is designed, for example, as an array of metallic islands deposited on a piezoceramic layer. The structured sacrificial layer can also have recesses, in particular such that it covers only a part of the green piezoceramic layer as a network structure.

According to one embodiment, the first metal in the first electrode layer can be present in a concentration of less than 100%. It is preferred that the first metal is present in the first electrode layer in a concentration of up to 80%.

It has been found that as the first metal, copper is particularly favorable because it softens at relatively low temperatures and thus gentle sintering of the piezoelectric multilayer component is possible in which the copper is well bonded to a piezoceramic layer. In addition, it has been found that copper, compared to other metals, such as palladium or platinum, diffuses relatively easily by piezoceramic, which promotes the production of a piezoelectric multilayer component described below with a hollowed-out, mechanical weakened region.

Instead of copper as the first metal, another metal may be used, such as silver or nickel.

According to one embodiment, the first electrode layer includes an additional second metal which is different from the first metal. The second metal is preferably palladium.

It is preferable that the second metal diffuses worse by a piezoceramic layer adjacent to the first electrode layer than the first metal. Thus, the diffusion of metal through the multilayer component is achieved primarily by the first metal, in particular by copper, which has a higher mobility due to piezoceramic when heated.

The second metal is preferably selected from palladium, beryllium, aluminum, manganese, zinc, tin, bismuth, nickel, cobalt, chromium, molybdenum, niobium, rubidium depending on what metal is used as the first metal in the first electrode layer.

It is favorable if, in the first electrode layer, the first metal is present in a higher concentration than the second metal. For example, the first metal is present in a concentration of 70% and the second metal is present in a concentration of 30% in the first electrode layer. In order for a diffusion to take place from the sacrificial layer to the first electrode layer, the concentration of the first metal in the first electrode layer must be lower than the concentration of the first metal in the sacrificial layer. Diffusion of the first metal reduces its concentration difference between the first electrode layer and the sacrificial layer, ie the concentration of the first metal in the sacrificial layer decreases.

The sacrificial layer may contain, in addition to the metals, an organic binder which preferably volatilizes before the sintering of the intermediate product by a suitable temperature treatment.

According to the invention, a second electrode layer is provided in the intermediate product which is applied to a further piezoceramic layer, the second electrode layer containing the same first metal as the first electrode layer and the sacrificial layer. This includes

The second electrode layer is the first metal in the same or almost equal concentration as the sacrificial layer. This is to prevent the diffusion of the first metal from the second electrode layer into the sacrificial layer as far as possible so that the second electrode layer suffers no or only very little material loss. This ensures that after the sintering of the intermediate product, the second electrode layer can function functionally as an electrode layer, ie it is suitable for the construction of electric fields.

It is preferred that the piezoceramic layers of the piezoelectric multilayer device contain a ceramic comprising lead zirconate titanate (PZT). It has been found that metals, in particular copper, can diffuse through a PZT ceramic during the sintering of the intermediate product with relatively low resistance. Thus, the diffusion process of a metal between two regions of the piezoelectric multilayer device in which the first metal is in different concentrations can be favored.

The invention also relates to a green body as an intermediate product in the production of a piezoelectric multilayer component, wherein stacked green sheets comprising a piezoelectric ceramic and interposed electrode layers are provided, and wherein a metal-containing sacrificial layer is arranged next to a first electrode layer Main component with a weight fraction greater than 50%. In this case, the first electrode layer contains a second metal, which is different from the first metal, as a secondary component with a weight fraction of less than 50%, the sacrificial layer containing the first metal as main component with a weight fraction which is greater than the weight fraction in the first electrode layer.

In addition to the piezoelectric multilayer component as intermediate product or as a green body, a process for the production of a piezoelectric multilayer component as final product or sintered product is also given.

In this process, the piezoelectric multilayer component produced as an intermediate product is sintered, the first metal at least partially diffusing from the sacrificial layer to the first electrode layer, thereby leaving cavities in the region formerly printed with the sacrificial layer, whereby the boundary surface between two piezoceramic layers between which the sacrificial layer was applied , Is mechanically weakened.

A piezoelectric multilayer component is also specified, which can be produced directly after the manufacturing process described here, in which the intermediate product is further processed.

The objects described are explained in more detail with reference to the following exemplary embodiments and figures. FIG.<dl id="dl0001"><dt>FIG</dt><dd>A longitudinal section of a piezoelectric multilayer component,</dd><dt>FIG</dt><dd>A portion of the stack of a piezoelectric multilayer device, </dd><dt>FIGS. 3a and 3b</dt><dd>Polarization cracking in a piezoelectric multilayer component,</dd><dt>FIG</dt><dd>A portion of the stack of a green piezoelectric multilayer device having a patterned metallic sacrificial layer,</dd><dt>FIGS. 5a to 5f</dt><dd>Various embodiments of a sacrificial layer,</dd><dt>Figures 6a to 6d</dt><dd>A section of the stack of a green piezoelectric multilayer component with different arrangement possibilities of a structured sacrificial layer with respect to the cross-sectional area of ​​the stack,</dd><dt>7a to 7d</dt><dd>A portion of the stack of a green piezoelectric multilayer component having different arrangement possibilities of one or more sacrificial layers,</dd><dt>FIG</dt><dd>A portion of the stack of a green multi-layer piezoelectric device having a repeating arrangement of first electrode layers, sacrificial layers and second electrode layers,</dd><dt>FIG</dt><dd>A portion of the stack of a green piezoelectric multilayer device having a repeating arrangement of first electrode layers and sacrificial layers, </dd><dt>FIG</dt><dd>A portion of the stack of a piezoelectric multi-layered piezoelectric ceramic device having a cavity area between two piezoceramic layers.</dd></dl>

<figref idrefs="f0001">FIG</figref> Shows the cross-section of a schematically illustrated piezoactuator which has a stack 1 of piezoceramic layers 2 and electrode layers 3 lying therebetween. On two longitudinal sides of the stack 1, external contacts are applied as external metallizations 5, which contact the ends of the electrode layers 3, which ends up to these longitudinal sides. Neighboring electrode layers of different polarity overlap in orthogonal projection, which runs parallel to the stack axis of the piezoelectric actuator. In the overlapping region, which is referred to as the active zone, an electric field leads to the generation of a deflection or expansion of a piezoceramic layer 2 present between these electrode layers. The region in which opposing adjacent electrode layers 3 do not overlap is referred to as an inactive zone. In this region, almost no deflection is caused by the piezoelectric effect. The material in the electrode layers 3 is selected such that they do not melt excessively during the sintering process or that the structure of the printed electrode layer remains essentially unchanged during and after the sintering.

<figref idrefs="f0001">FIG</figref> Shows a section of a stack 1 of a piezoelectric actuator in which a plurality of piezoceramic layers 2 can be stacked one above the other between mutually adjacent electrode layers 3. Such a section also serves as the basis for a construction for the embodiments of a piezoelectric actuator described in the following, since additional electrode layers 3 and / or sacrificial layers can be applied between the additional piezoceramic layers 2, which diffusion processes can stimulate diffusion processes through their different material compositions.

<figref idrefs="f0001">FIG. 3a</figref> Shows how a crack 6 connects several electrode layers 3, in particular opposing electrode layers 3 of a piezoelectric actuator.

The inventors have determined that the reliability of a piezoelectric actuator is decisively dependent on the control of possible cracks. In the case of thermal processes such as sintering at temperatures between 800 and 1500 ° C., metallization and soldering as well as in the polarization of the sintered piezo actuator, mechanical stresses arise due to the different elongation in the active and inactive zone, resulting in so-called relief cracks and / or polarization cracks Of the piezoelectric actuator. These run in the inactive zone along or in an electrode layer 3. During the transition into the active region, these cracks can bend. If these cracks bridge at least two electrode layers, short circuits can occur which can lead to the failure of the piezoelectric actuator. In contrast, cracks which run parallel to the internal electrodes are virtually no danger to the life of piezoelectric actuators.

<figref idrefs="f0001">3b</figref> Shows a harmless course of a crack 6 in the stack 1 of a piezoelectric actuator. In this case, the crack runs essentially parallel to an electrode layer 3 or to a piezoceramic layer 2, so that the crack does not connect opposing electrode layers and thus also does not cause short circuits.

An idea to avoid harmful cracks as in <figref idrefs="f0001">FIG. 3a</figref> Is to use adjacent metallic layers of different materials to augment diffusion processes that are to take place at higher temperatures during the sintering process due to the different compositions of these metallic layers. During the diffusion process, a metallic layer or a component of an alloy of this layer should lose more material than the other. In this metallic layer cavities will be formed, which will lead to the mechanical attenuation of this layer. Polishing cracks or other cracks are therefore preferably formed in the mechanically weakened metallic layer and propagate only within the layer.

<figref idrefs="f0002">FIG</figref> 1 shows a longitudinal section of a schematically illustrated section of a stack 1 of a piezoactuator, wherein a sacrificial layer 4 is arranged on a piezoceramic layer 2, the sacrificial layer being arranged between a first electrode layer 3a and a second electrode layer 3b, and these electrode layers being arranged oppositely and in the stacking direction . In this context, the term "adjacent to the stacking direction" means that there are no further functional electrode layers between the first electrode layer 3a and the second electrode layer 3b.

For example, a material with a composition (1-x) Cu / x Pd can be used in a given number of first electrode layers 3a instead of exclusively copper, where 0 &lt;x &lt;1. This material may be either a mixture of copper and palladium powder, or an alloy of these two metals. The first electrode layer 3a thus contains a mixture of copper and palladium, whereas the sacrificial layer 4 as a metal is preferably only copper. Alternatively, another metal, such as silver, may be used instead of copper. The first electrode layer 3a contains, for example, a mixture or an alloy of silver and palladium. The sacrificial layer 4 preferably contains only silver.

The difference in the composition of the first electrode layer 3a and the sacrificial layer 4 will stimulate diffusion processes at higher temperatures. It has been found that the copper exhibits a higher mobility in piezoelectric ceramics based on PZT than palladium. This leads to diffusion only taking place in one direction, namely from the sacrificial layer 4 of pure copper into the first electrode layer 3a containing copper and palladium. The first electrode layer 3a containing copper and palladium thus plays the role of a copper core. The loss of material in the sacrificial layer 4 in direct proximity to the first copper-palladium electrode layer 3a leads to the formation of cavities which weakens the interface between the previously present sacrificial layer and surrounding piezoceramic layers 2. Thus conditions are created for the formation and propagation of controlled cracks according to<figref idrefs="f0001">3b</figref>, Which essentially extend within the interface and thus parallel to piezoceramic layers 2.

The proportion of cavities in the sacrificial layer 4 can be controlled by the composition of the first electrode layers 3a and the sacrificial layer 4, the thickness of the layers as well as by a spatial structuring of the sacrificial layer 4.

The sacrificial layer 4 is preferably applied as a pattern of metallic islands or as a metal layer with a pattern of recesses on a piezoceramic layer 2. Preferably, the metallic region or the metallic regions of such a structured sacrificial layer 4 contains only the first metal, in this example copper.

Preferably, the patterned sacrificial layer 4 is applied by means of screen printing, sputtering or spraying onto the piezoceramic layer.

The piezoceramic layers contain, for example, a ceramic having a composition according to the following formulas:         (Pb_xNd_y) ((Zr_1-zTi_e.g.,) "_1-a Ni_a)O₃, in which<ul><li>0.90 ≤ x ≤ 1.10;</li><li>0.0001 ≤ y ≤ 0.06;</li><li>0.35 ≤ z ≤ 0.60;</li><li>0 ≤ a ≤ 0.10.</li></ul>

Preferably, 10% of the total number of electrode layers in the piezoelectric actuator are first electrode layers 3a, ie they contain a first metal in a lower concentration than a sacrificial layer 4.

A more detailed description of a preferred composition of the first electrode layer 3a follows. Thereby

Copper is present in the first electrode layer 3a to a weight fraction of 99.9% to 70%, in particular to a proportion of 97% to 75%. The remainder of the first electrode layer contains palladium as the metal, organic binders not being included here being included. In this case, either an alloy of copper and palladium or a mixture of copper powder and palladium powder is used.

Copper particles in the first electrode layer 3a and / or in the sacrificial layer 4 have diameters of 0.1 to 10 μm, preferably 0.4 to 1.5 μm.

Palladium particles in the first electrode layer 3a likewise have diameters of 0.1 to 10 μm, preferably 0.4 to 1.5 μm. Other metal particles, for example silver particles, can also have these sizes.

The particle sizes in an alloy in the first electrode layer 3a, that is, in a combination of various metals which are not separately in the form of different powders, may also have the above-mentioned dimensions.

The first electrode layer 3a is also preferably applied to a piezoceramic layer 2 by means of screen printing, sputtering or spraying. Advantageously, the same printing process can be used, which is used for the production of the sacrificial layer 4.

The thickness of a first electrode layer 3a in the non-sintered state of the piezoelectric actuator is preferably between 0.1 and 20 μm, preferably 1.0 and 10 μm.

The proportion of first electrode layers 3a can be between 1% and 100% of the total number of electrode layers of the piezoelectric multilayer component. In a preferred embodiment, the proportion of the first electrode layers is in the range between 5% and 15%. Preferably, at least one sacrificial layer 4 is located next to each first electrode layer 3a.

<figref idrefs="f0002">FIG</figref> Shows a plan view of a sacrificial layer 4 which covers the entire surface of a piezoceramic layer 2. Due to the material loss of this sacrificial layer 4 during sintering, cavities are produced. Given a suitable choice of the material composition of the sacrificial layer, almost the entire quantity of the first metal can diffuse during the sintering to the first electrode layer. However, the adjacent piezoceramic layers 2 can not bond firmly to one another, so that the boundary surface between these piezoceramic layers 2 is mechanically weakened and can thus serve as a desired breaking layer during the operation of the piezoelectric actuator.

<figref idrefs="f0002">FIG. 5b</figref> Shows a plan view of a patterned sacrificial layer 4 constructed as an array of metallic islands 4a covering only a part of the surface of a piezoceramic layer 2. The metallic islands 4a are circular here, but they can each have a different contour, for example a polygonal contour. The metallic islands 4a are preferably applied in a regular pattern on the piezoceramic layer 2, so that they each have the same distances from each other. However, such a regular structure is not a requirement.

<figref idrefs="f0002">FIG. 5c</figref> Shows a plan view of a structured, metallic, sacrificial layer 4 with recesses or holes 4b so that the layer 4 covers only a part of the surface of a piezoceramic layer 2. FIG. The printed structure is a negative image of the structure of the previous figure. The holes 4b are preferably circular and spaced apart at regular intervals. The region of the piezoceramic layer printed with the metallic layer 4 can extend as far as the edge of the piezoceramic layer. However, since the printed first metal migrates later during the sintering of the green stack, a cavity remains at the edge region of the piezoceramic layer, so that externally stacked external metallizations no longer have any electrical contact with the inner region of the stack 1 on the plane of the migrated first metal Respectively.

<figref idrefs="f0002">5d</figref> Shows an embodiment of the patterned metallic sacrificial layer 4, in which the metallic islands 4a are square.

<figref idrefs="f0002">FIG. 5e</figref> Shows a metallic sacrificial layer 4 which is applied as a network structure on a piezoceramic layer 2. Thus, the first metal is applied to the piezoceramic layer 2 in a contiguous structure including square recesses 4b. In contrast to the<figref idrefs="f0002">FIG. 5c</figref> The recesses 4b are square instead of circular and the metallic area does not reach as far as the edge of the stack 1.

<figref idrefs="f0002">5f</figref> Shows a metallic sacrificial layer 4, which is applied as an arrangement of concentric, frame-shaped metallic regions on a piezoceramic layer 2. The metallic regions can have circular or square outlines. They can be understood as frame-shaped metallic islands having a common center. This embodiment can be embodied as a combination of metallic islands 4a according to FIG<figref idrefs="f0002">FIG. 4b</figref> And recesses 4b according to FIG <figref idrefs="f0002">4c and 4e</figref> Be understood.

The <figref idrefs="f0002">Figures 6a to 6d</figref> Represent various positions of an array of metallic islands 4a on a surface of a piezoceramic layer 2. For example, a first array of metallic islands having a substantially rectangular outline may extend with one side of this rectangular outline to a longitudinal side of the stack 1<figref idrefs="f0002">FIG. 6a</figref>) Have a distance from the opposite longitudinal side of the stack with an opposite side of the outline of the island arrangement. Instead, opposing sides of a rectangular outline of an array of metallic islands can extend to two opposing longitudinal sides of the stack and be offset back against the further longitudinal sides (<figref idrefs="f0002">FIG. 6b</figref>). Additionally or alternatively, the array of metallic islands may have a contour in the form of a parallelogram so that two corners of this contour extend to two corners of the stack (<figref idrefs="f0002">FIG. 6c</figref>). Alternatively, the arrangement of metallic islands can be located only in the inner region of the surface of a piezoceramic layer so that no part of the contour of the arrangement extends to one longitudinal side of the stack.

The <figref idrefs="f0003">7a to 7d</figref> Show the longitudinal section of a section of a stack 1 of a piezoactuator with different arrangement possibilities of first electrode layers 3a and sacrificial layers 4, which are enclosed in each case by second electrode layers 3b in the stacking direction. The combinations of first electrode layers 3a and outer layers 4 shown here can be present in the stack as often as desired and do not all need to be enclosed by second electrode layers 3b.<figref idrefs="f0003">7a</figref> Shows an arrangement in which a first electrode layer 3a containing a first metal in a lower concentration is arranged as the first metal in a sacrificial layer 4, between two adjacent sacrificial layers 4 in the stacking direction, each having a structure according to one of FIGS <figref idrefs="f0002">FIGS. 5a to 6d</figref> Respectively. Two first electrode layers 3a are adjacent to one another in the stacking direction. Each first electrode layer 3a is enclosed in the stacking direction by two sacrificial layers 4. An "inner" sacrificial layer adjoins in the stacking direction to two first electrode layers 3a. Two "outer" sacrificial layers each adjoin a first electrode layer 3a and a second electrode layer 3b, the metal part of which is preferably entirely formed from the first metal. For example, the arrangement consisting of the sacrificial layer, the first electrode layer, the sacrificial layer, the first electrode layer and the sacrificial layer continues in the stacking direction. The first electrode layer 3a preferably contains a mixture of a first metal, for example copper, and a second metal, for example palladium. The copper of two sacrificial layers 4, between which a first electrode layer 3a is arranged, can diffuse to this first electrode layer and thereby leave cavities. The first electrode layer 3a thus serves as a sink for the first metal of two sacrificial layers.

<figref idrefs="f0003">7b</figref> Shows an exemplary embodiment in which a sacrificial layer 4 with the first metal is positioned between a first 3a and a second electrode layer 3b. In relation to the stacking direction of the stack 1, however, the first electrode layer 3a or the second electrode layer 3b can be located above or below the sacrificial layer 4. Arrangements of such triple groups can be spaced apart by means of several piezoceramic layers 2. In the exemplary embodiment shown here, two piezoceramic layers are always present between two functional electrode layers.

According to <figref idrefs="f0003">7c</figref> A sacrificial layer 4 is arranged between two opposing first electrode layers 3a, which are adjacent in the stacking direction. Here, two first electrode layers 3a "divide" a sacrificial layer. Thus, the first metal from the sacrificial layer 4 can simultaneously diffuse into two adjacent first electrode layers 3a during the sintering of the piezoelectric actuator. Here, too, the first electrode layers 3a preferably contain a second metal which is poorly diffusible by a piezoceramic, such as, for example, palladium.

According to <figref idrefs="f0003">7d</figref> First electrode layers 3a and second electrode layers 3b alternate in the stacking direction on a regular basis, the first electrode layers 3a each being surrounded by two sacrificial layers 4.

<figref idrefs="f0003">FIG</figref> Shows a portion of a stack 1 of a green piezoelectric multilayer device in which a pair of first electrode layer 3a and sacrificial layer 4 are provided at regular intervals in the stack. The remaining electrode layers are second electrode layers 3b.

A plurality of second electrode layers 3b may be replaced by pairs of first electrode layers 3a and sacrificial layers 4. However, only a single such pair may be provided in a stack.

<figref idrefs="f0004">FIG</figref> Shows a section of a stack of a non-inventive green piezoelectric multilayer component in which only first electrode layers 3a and outer layers 4 are present and second electrode layers are missing. A sacrificial layer 4, the metal part of which is preferably made entirely of copper, is arranged between some opposing first electrode layers 3a, which, for example, contain mainly copper and a palladium. In the stacking direction, not every first electrode layer 3a is adjacent to a sacrificial layer 4. The sacrificial layers 4 can be present in any number in the stack. For example, only one sacrificial layer 4 may be provided, or a sacrificial layer 4 may be provided next to each first electrode layer 3a.

<figref idrefs="f0004">FIG</figref> Shows a section of a stack 1 of a sintered piezoelectric multilayer component, in which cavities 7 are formed at the boundary between two piezoceramic layers 2, the first metal of the sacrificial layer being present before the sintering at the location of the cavities 7. During sintering, the first metal of the sacrificial layer migrated by diffusion to a first electrode layer 3a with a lower concentration of the first metal. By structuring the sacrificial layer, eg according to the<figref idrefs="f0002">FIGS. 5b to 5f and 6a to 6d</figref> The cavities 7 can be generated specifically at specific positions of the interface.

The diffusion process can be achieved on the one hand by a suitable adjustment of the concentration difference between the sacrificial layer 4 and an adjacent first electrode layer 3a with the lower concentration of the first metal.

On the other hand, the diffusion process can be achieved by suitable temperature control during sintering or heating of the green stack. The<figref idrefs="f0004">FIG</figref> Shows existing cavities between a first electrode layer 3a and a second electrode layer 3b. Diffusion of the metal of the sacrificial layer occurred almost exclusively in the direction of the first electrode layer 3a since the second electrode layer 3b contains a concentration of the first metal which is equal to or nearly equal to the concentration of the first metal in the sacrificial layer. There was thus no appreciable concentration gradient between the sacrificial layer 4 and the second electrode layer 3b, which would stimulate a diffusion process of the first metal between them.

Reference list

<dl id="dl0002" compact="compact"><dt>1</dt><dd>Stack of piezoceramic layers and electrode layers</dd><dt>2</dt><dd>piezoceramic layer</dd><dt>3</dt><dd>electrode layer</dd><dt>3a</dt><dd>first electrode layer</dd><dt>3b</dt><dd>second electrode layer</dd><dt>4</dt><dd>sacrifice</dd><dt>5</dt><dd>External contact</dd><dt>6</dt><dd>crack</dd><dt>7</dt><dd>Cavities</dd></dl>