EP1794819A1

EP1794819A1 - Solid-state actuator, especially piezoceramic actuator

Info

Publication number: EP1794819A1
Application number: EP05763994A
Authority: EP
Inventors: Karl Lubitz; Thorsten Steinkopff
Original assignee: Siemens AG
Current assignee: Siemens AG
Priority date: 2004-09-30
Filing date: 2005-08-02
Publication date: 2007-06-13
Also published as: JP2008515213A; CN101053088A; DE102004047696B4; DE102004047696A1; WO2006034905A1; US20070252478A1

Abstract

The invention relates to a solid-state actuator (1), especially a piezoceramic actuator, which comprises a support layer (2) to which at least one actuator layer (3), especially a piezoceramic layer, is applied, said actuator layer (3) being disposed between contact electrodes (4, 5). In order to avoid a creep behavior of the solid-state actuator, the resistivity of the actuator layer (3) is rated between 1⋅108 Ωm to 1⋅1010 Ωm and/or an actuator control device (6) for applying a control voltage to the contact electrodes (4, 5) is provided and the maximum control voltage is selected in such a manner that the maximum mechanical voltage in the solid-state actuator is below the coercive voltage.

Description

description

Solid state actuator, in particular piezoceramic actuator

The invention relates to a solid-state actuator, in particular a piezoceramic actuator, with a carrier layer on which at least one actuator layer, in particular a piezoceramic layer, is applied, wherein the actuator layer is arranged between contact electrodes.

Solid state actuators and in particular piezoceramic actuators are known, which in the simplest case are composed of the composite of an actuator material and a carrier layer or of a multiplicity of, for example, joined disks of piezoceramic material. By applying contact electrodes to the actuator layer or actuator layers on both sides and applying a voltage to the contact electrodes, an electric field can be built up between them so that an electric field acts on the piezoceramic material, resulting in that the piezoceramic material undergoes a change in length.

The solid state actuator may be formed, for example, as a piezoelectric bending transducer. In this composite, referred to in the simplest case as a bimorph, the piezoceramic sacrificial layer is arranged on a non-piezoelectric or non-controlled piezoelectric carrier layer, wherein the actuator layer is usually made of a PZT ceramic, which is doped lead zirconate titanate is. Bending transducers are usually clamped on one side, wherein the force or deflection generated at the free end of the solid-state actuator is used as an actuatoric property. If the bending transducer is controlled with an electric field in the thickness direction, then the transducer shortens in its transverse direction, as a result of which it deflects at its tip in the direction of the actuator layer. In addition, further piezoelectric bending transducers are known, which may differ in design, in the type of their structure, in the selection of the carrier material and other criteria. For example, solid-state actuators designated as trimorphs consist of two piezoelectric actuator layers, which are connected to the carrier layer as the central layer and, for example, are driven alternately. In addition, multi-layer bending transducers are also known which have no carrier layer and consist solely of a large number of piezoceramic actuator layers. In each case only one half is electrically driven in order to generate a deflection.

The bending transducers described above, it is common that they after a quick, electrical control of the

Contact electrodes have an immediate, over time by the resonance frequency determined actuator response, but then also show a strong creep behavior, so that the deflection or the force increase over a long period of time. In this case, the amount of "after-creep" can reach up to 20% of the total deflection of the bending transducer The duration of after-creep can amount to hours or even days if appropriately controlled In practice, this has the disadvantage that creeping occurs during application and when switching off As a usable deflection or force stroke, therefore, only the short-term, immediate stroke without additional creep is usually used.

The object of the present invention is to provide a solid-state actuator, which does not have the disadvantages mentioned above or at least only to a reduced extent.

The object is achieved by solid state actuators having the features of claims 1, 2 or 3. Advantageous Ausges tions are described in the dependent claims benrie.

It has surprisingly turned out, according to a variant of the invention, that the phenomenon of creep can be significantly reduced if the electrical conductivity of the material forming the actuator layer is greater than that of conventionally used materials, such as lead zirconate titanate (PZT). increases or the resistivity is reduced. According to the invention, the specific resistance of such an actuator layer, which is designed in particular as a piezo-ceramic layer, is in the order of 1-10 ⁸ Ωm to 1-10 ¹⁰ Ωm. The specific resistance of an actuator layer according to the invention is thus a few orders of magnitude below the specific resistance for a typical piezoceramic layer. Thus, the speci¬ fic resistance of soft PZT is about 1-10 ¹² Ωm.

The advantage that can be achieved by increasing the conductivity is that the stroke that can be achieved in a conventional solid-state actuator or the achievable deflection, including the deflection that can be achieved by the creeping process, can be realized in a considerably shorter time. In other words, this means that with the solid-state actuator according to the invention-compared to previous solid state actuators-the same deflection can be achieved in a shorter time, so that the solid-state actuator can be operated at higher clock rates. On the one hand, this has the consequence that not only the short-term stroke without the additional creeping process, but the physically possible stroke of the solid-state actuator can be used as the usable deflection or power stroke. This simplifies the control of the solid-state actuator, since it is no longer necessary to plan for creeping during installation and when switching off an electrical voltage as an additional tolerance. The same advantages can be achieved with a second variant of a solid state actuator according to the invention, in which a Aktoransteuermittel is provided for applying a drive voltage to the contact electrodes and in which the maximum drive voltage is chosen such that in the Festkörpe¬ raktor the maximum mechanical stress below the Koer - zitivspannung lies. The mechanical stresses in the piezoceramic materials used in the rule are in the range of the so-called coercive voltage values in which a maximum domain switching occurs under the influence of the mechanical stresses. This is referred to as so-called "ferro-elastic behavior." This second variant is based on the surprising finding that creep is caused, at least in part, by domain switching or ferroelastic processes in those regions of the bender in which the mechanical force is generated It is known that the mechanical stresses vary along the thickness of an actuator layer, while they are constant in the longitudinal direction thereof.

Growth processes and are characterized by a certain amount of time. By avoiding the ferro-elastic domain switching, the activity, i. the bending, of the solid-state actuator is not delayed.

The above-described advantages of the invention can also be achieved by a solid state actuator according to the invention, in which the features described according to the first and second variants are combined with one another. Accordingly, the solid-state actuator according to the invention according to the third variant is characterized in that the specific resistance of the actuator layer in the order of 1-10 ⁸ Ωm to 1-10 ¹⁰ Ωm and an Aktoransteuermittel is provided for applying a drive voltage to the contact electrodes and the maximum Drive voltage is chosen such that in the solid-state actuator, the maximum mechanical stress is below half the coercive voltage. In a preferred embodiment, a solid-state actuator provided with the above features represents a piezoelectric bending transducer, which is arranged with one end on or in a fastening means, so that only the other end can experience a deflection.

In a further embodiment of the invention, the relationship between the drive voltage and the mechanical stress in the solid-state actuator is determined by a calculation or is shown in a table, e.g. stored in the Aktoransteuermit¬ tel.

The increase in the electrical conductivity of the material of the actuator layer can be achieved according to an embodiment of the invention by additionally doping the actuator material with mono-, di- or trivalent cations. As Aktor- starting material lead zirconate titanate is preferred. In one embodiment, the monovalent cations on the A-site of the perkoswit cell lead to acceptor doping. In another embodiment, the divalent or trivalent cations on the B-site of the perkoswit cell also lead to acceptor doping. Also conceivable is a combination of the two named possibilities of acceptor doping.

In a further embodiment, the solid-state actuator is designed as a so-called trimorph, in which the carrier layer is arranged between two actuator layers.

In a further expedient embodiment, the carrier layer is formed as an actuator layer, in particular a piezoceramic layer, so that the solid-state actuator constitutes a multilayer actuator comprising at least two actuator layers.

In another embodiment, the solid-state actuator may have a multiplicity of actuator layers for forming a multilayer actuator, wherein the solid state components in the interior of the layer stack ordered contact electrodes by the control means eben¬ if the formation of equipotential surfaces are controlled. The electrically highly conductive, arranged in the interior of the Schicht¬ stack electrodes are preferably formed of silver or a silver alloy and act as equipotential surfaces, so that they compensate for a significant part of the electric field distribution by appropriate La¬. In addition, the silver of the electrodes diffuses into the adjacent piezoceramic actuator layers, whereby further free charge carriers are present in the ceramic, so that the conductivity advantageously further increases. This effect is particularly pronounced due to the presence of a multiplicity of electrodes. A multi-layer actuator designed in this way has compared to those used in the prior art

Solid state actuators the same advantages as described above. In particular, a significant Reduk¬ tion of creeping is observed.

In one embodiment, the actuator layers of the multilayer actuator have a thickness in the range between 10 μm to 30 μm, in particular 20 μm. A multilayer actuator with Aktor¬ layers of said layer thickness is unchanged in its Gesamtdi¬ over the known multilayer actuators. In other words, this therefore means that a multilayer actuator according to the invention has a correspondingly larger number of individual actuator layers, since the thicknesses of conventional actuator layers are in the range of 80 μm and above.

Further features of the solid-state actuator according to the invention are explained in more detail below with reference to the figures. It shows:

1 shows a solid-state actuator according to the invention, which is designed as a bimorph bending transducer, FIG. 2 is a graph showing the change in length of the layers of the solid-state actuator shown in FIG. 1 along the z-axis. FIG.

FIG. 3 is a graph showing the stress along the z-axis of the solid state actuator shown in FIG. 1. FIG.

4 shows a diagram which represents the deflection of a solid-state actuator as a result of a drive signal at different electrical conductivities of the contactor layer of the solid-state actuator,

5 shows a diagram which shows a drive according to the invention of a solid state actuator, and

6 shows a multilayer actuator according to the invention in comparison to a multilayer actuator known from the prior art.

In Fig. 1, a solid state actuator 1 according to the invention is shown in cross section. This has a carrier layer 2 of an electrically insulating material and an actuator layer 3 applied thereon of a piezoceramic material, eg lead zirconate titanate. Arranged on both sides of the actuator layer 3 are contact electrodes 4, 5, to which an electrical voltage can be applied, so that between the contact electrodes 4, 5 there is an electric field. The mechanical structure of the solid state actuator 1 according to the invention does not differ in principle from known solid state actuators. In order to avoid a pronounced creep behavior during the application or disconnection of the electrical voltage, changes to the piezoceramic material as well as alternatively or additionally to the control of the solid-state actuator 1 are performed in comparison to known arrangements. By applying a voltage to the contact electrodes 4, 5, the actuator layer 3 expands along its z-axis, while in the x-direction a shortening occurs, so that the solid-state actuator bends upwards. The change in length Δl / lo taking place in the interior of the carrier layer and of the actuator layer 3 is shown in FIG. While the carrier layer 2 undergoes an elongation until the so-called neutral fiber 7 is reached, the actuator layer 3 is compressed. Since the material properties of the carrier layer 2 and of the actuator layer 3 are different, the mechanical stress experienced when passing through the zero point changes abruptly.

According to the equation shown in FIG

acts on the majority of the support layer 2, a train, wherein in the interior of the actuator layer 3, a pressure is applied. Due to different Young 'shear modules y2 and y3 results in the transition between the carrier layer 2 and the actuator layer 3, a jump in the pressure load, which continues to be reflected in a changed slope.

Thus, an inhomogeneous mechanical stress, which results in an inhomogeneous distribution of the electric field in the actuator layer 3, results via the z-axis. When a voltage is applied to the contact electrodes 4, 5, a constant, homogeneous electric field does not arise in the interior of the actuator layer 3, but rather a linear field dependence with a parabolic potential distribution. In order to set a state of equilibrium, charges therefore have to flow in the interior of the actuator layer. It has been found that the part of the creep attributable to the charge balance can be reduced by replacing one after the other

State of the art as possible insulating piezoceramic a Ceramics with higher but defined conductivity is used. In the course of further investigations, it has been found that a specific resistance of the actuator layer 3 in a magnitude of 1-10 ⁸ Ωm to 1-10 ¹⁰ Ωm allows a sufficiently fast equipotential bonding, so that the creeping of the bending transducer 1 ceases from Fig. 1 can almost avoided wer¬ the. The increase in the electrical conductivity of the actuator layer by several orders of magnitude can be achieved in a known manner by low acceptor doping in the material of the actuator layer, without at the same time worsening the other piezoelectric properties. Lead zirconate titanate, in which monovalent cations on the A-site of the percolosite cell, for example sodium, copper or silver, or, alternatively or additionally, bivalent or trivalent cations on the B, is also suitable as starting material for the actuator layer Placement, such as chromium, iron, Man¬ gan, doped.

The effects of different electrical conductivities of the actuator layers are shown in FIG. 4. At a time t ₀ , a voltage is applied to the contact electrodes 4, 5 of the solid-state contactor 1. Because of this, a deflection of the bending transducer takes place. An actuator layer (ceramic) with low conductivity generates the lowest deflection. A ceramic with a low conductivity, as it is piezoelectric ceramics according to the prior art, reached at time ti Hi a stroke, the now-a- symptotisch a Entwert H approaches. ₃ The increase in deflection beyond the Hi value is referred to as creep behavior. At the time t ₃ , a deflection H2 is achieved. The maximum possible deflection H ₃ can only be achieved if the drive signal retains its value shown in the figure. In contrast, a ceramic with high conductivity according to the invention has the deflection H2 at the time ti. The further possible deflection between H ₂ and H ₃ is irrelevant in practice. From the illustration shown, it can be seen that a bending transducer can achieve a much higher deflection within the same time or, alternatively, can be clocked in a shorter time with a required deflection.

With additional free charge carriers in the actuator layer thus the creep effect of a solid state actuator can be reduced. The creep can however be influenced by another effect, the so-called domain switching. The domain switching, i. the change of direction of elementary

Dipoles can be induced both electrically and mechanically. In the case of conventional piezoceramic layers, the maximum possible mechanical stress T _max is in the range of the so-called coercive voltage values in which the maximum domain switching occurs under the influence of the mechanical stresses. This is called ferro-elastic behavior. In order to avoid the influence of the domain switching, the drive voltage is therefore limited in such a way that in a controlled solid-state actuator the maximum mechanical stresses remain clearly below the coercive voltages (FIG. 5). Corresponding information can be stored in a Aktoransteuermittel about a calculation or stored values in ei¬. Also conceivable is the use of other piezoceramics, which already have a higher coercive stress due to their material properties. Suitable materials are those whose coercive stress is higher than 25 MPa.

If the solid-state actuator is designed as a multilayer actuator, then the creep behavior can already be achieved by a changed physical structure. FIG. 6a shows a multi-layer actuator (consisting of three layers 3), as known from the prior art. Each of the actuator layers 3 in this case has a layer thickness of about 80 microns and above. The electrodes arranged in the layer stack are also driven by the actuator drive means. In contrast, Fig. 6b shows a multilayer actuator according to the invention, in which the layer thicknesses of respective Aktor¬ layers 3 in the range between 10 .mu.m to 30 .mu.m, preferably 20 microns, are. The electrodes located in the interior of the layer stack are driven by the actuator drive means and have a connection to the contact electrodes 4, 5 on the outer sides of the multilayer actuator. The electrodes located in the interior of the multilayer actuator, which are preferably formed from silver or a silver alloy, thus represent equipotential surfaces which can compensate for the greater part of the electric field distribution by means of corresponding charges. In addition, the silver of the electrodes diffuses into the adjacent piezoceramic actuator layers, as a result of which further free charge carriers are present in the ceramic, so that the conductivity advantageously further increases. This effect is particularly pronounced due to the presence of a large number of electrodes. In this way it is ensured that an improvement in the creep behavior occurs. By combining with the above-described improvements, the creep behavior can be further optimized.

Example:

Using the example of a typical bending transducer, the inventions according to the invention should be illustrated again. A bending transducer consists of two piezoceramic layers (44 × 7.2 × 0.26 mm ³ ), which are applied on both sides to an insulating carrier layer. If one of the actuator layers is driven at 200 V, a current of 0.24 nA flows at a specific resistance of 1-10 ¹² Ωm typical for soft PZT. The time constant for internal transloading processes is in the range of 1 to 1000 seconds. If the specific resistance of the ceramic material is reduced by three orders of magnitude by appropriate doping, the time constant responsible for the creep falls in the millisecond to second order

il customer area. At the same time, the continuous current of the bending converter still remains well below the limit value of 1 μA.

Claims

claims

1. solid state actuator (1), in particular piezoceramic actuator, with a carrier layer (2) on which at least one actuator layer (3), in particular a piezoceramic layer, is applied, wherein the actuator layer (3) is arranged between contact electrodes (4, 5), characterized in that the specific resistance of the actuator layer (3) is of the order of 1-10 ⁸ Ωm to 1-10 ¹⁰ Ωm.

2. solid-state actuator, in particular piezoceramic actuator, with a carrier layer (2) on which at least one actuator layer (3), in particular a piezoceramic layer is applied, wherein the actuator layer (3) between contact electrodes (4,5) is arranged, characterized in that an actuator drive means (6) for applying a Ansteuerspan¬ voltage to the contact electrodes (4,5) is provided and the maximum drive voltage is chosen such that in the solid state actuator, the maximum mechanical stress is below the coercive voltage.

3. solid-state actuator, in particular piezoceramic actuator, with a carrier layer (2) on which at least one actuator layer (3), in particular a piezoceramic layer is applied, wherein the actuator layer (3) between contact electrodes (4,5) is arranged, characterized in that - The specific resistance of the actuator layer (3) in the order of 1-10 ⁸ Ωm to 1-10 ¹⁰ Ωm, and

- An Aktoransteuermittel (6) for applying a Ansteuerspan¬ voltage to the contact electrodes (4,5) is provided and the maximum drive voltage is chosen such that in the solid-state actuator, the maximum mechanical stress is below the coercive voltage.

4. Solid state actuator according to one of claims 2 or 3, characterized in that the relationship between the drive voltage and the mechanical nischen voltage in the solid-state actuator (1) is stored in a table or is determined by a calculation.

5. Solid state actuator according to one of claims 1 or 3, characterized in that the actuator layer (3) is formed from lead zirconate titanate (PZT) and is additionally doped with mono- or di- or tri-valent cations.

6. Solid state actuator according to claim 5 or 6, characterized in that the monovalent cations are incorporated on the A-site of the perkoswit cell and give an acceptor doping.

7. solid-state actuator according to claim 5, characterized in that the divalent or trivalent cations are incorporated on the B-site of Per¬ koswit cell and an acceptor doping erge¬ ben.

8. Solid state actuator according to one of the preceding claims, characterized in that the carrier layer (2) is arranged between two Aktorschichten.

9. solid-state actuator according to one of the preceding claims, characterized in that the carrier layer (2) as an actuator layer (3), in particular piezoceramic layer is formed.

10. Solid-state actuator according to one of the preceding claims, characterized in that it has a multiplicity of actuator layers (3) for the formation of a multilayer actuator and those in the interior of the Layer stack arranged contact electrodes (8) by the Aktoransteuermittel (6) are driven to form equipotential surfaces.

11. Solid state actuator according to claim 10, characterized in that the actuator layers (3) of the multilayer actuator have a thickness in the range between 10 μm to 30 μm, in particular 20 μm, aufwei¬ sen.

12. Solid state actuator according to one of the preceding claims, characterized in that it represents a piezoelectric bending transducer.