ELECTROMECHANICAL ACTUATOR HAVING CERAMIC INSULATION AND METHOD FOR PRODUCTION THEREOF

Abstract

The present disclosure relates to an electromechanical actuator, having a stack arrangement made of ceramic basis material having electromechanical properties and electrodes as well as a ceramic insulation for operation/use of the actuator in a humid environment. To ensure a long service life of the actuator with increased electromechanical expansion, an exemplary structure of the ceramic insulation has a smaller average grain size than the structure of the ceramic basis material. A method for the production of an actuator having ceramic insulation and a method for controlling such an actuator are also disclosed.

Claims

1. Electromechanical actuator comprising: a stack arrangement made of ceramic basis material having electromechanical properties; electrodes; and a ceramic insulation configured for operation of said actuator in a humid environment, wherein a structure of said ceramic insulation has a smaller average grain size than the a structure of said ceramic basis material.

2. Actuator according to claim 1, wherein a ratio of average grain size of the ceramic insulation structure to that of the ceramic basis material is selected to be one or more of ¾ or less, ½ or less, and/or ⅓ or less.

3. Actuator according to claim 1, wherein an average grain size of said ceramic insulation structure is selected to be one or more of 20 μm or less, 5 μm or less, and/or 1 μm or less.

4. Actuator according to claim 1, wherein an average grain size of the ceramic basis material decreases continuously towards said ceramic insulation, and/or beyond said ceramic insulation.

5. Actuator according to claim 1, wherein said ceramic insulation is made of at least one or more of electromechanical, piezoelectric, and/or piezoceramic, material.

6. Actuator according to claim 1, wherein said ceramic insulation is impermeable to water vapor/moisture.

7. Actuator according to claim 1, wherein said ceramic insulation is made of at least one or more layers, where a total thickness of said ceramic insulation is at least one or more of 500 μm or less, 100 μm or less, and/or 60 μm or less.

8. Actuator according to claim 1, wherein said ceramic insulation has substantially a same or a different material composition as/than said ceramic basis material.

9. Actuator according to claim 1, comprising: two exterior electrodes for contacting said electrodes in said stack arrangement, arranged on a same outer surface or on two different outer surfaces of said actuator, where at least one electrode-free outer surface of said actuator includes said ceramic insulation.

10. Actuator according to claim 1, where said ceramic basis material and said electrodes are arranged along a stack axis and said stack arrangement comprises: two end faces aligned perpendicular to said stack axis and at least one side surface extending between said end faces, where said ceramic insulation on said at least one side surface extends from one end face to the other end face.

11. Method for production of an electromechanical actuator, the method comprising: A: forming a stack arrangement from electrodes and from a ceramic basis material having electromechanical properties; and B: providing said stack arrangement with a ceramic insulation such that a structure of said ceramic insulation has a smaller average grain size than a structure of said ceramic basis material.

12. Method according to claim 11, the method comprising at least one of the following partial steps: A1: forming said stack arrangement from said electrodes and from green tapes made of said ceramic basis material; and/or A2: sintering said stack arrangement so that said ceramic basis material is transformed into a solid ceramic structure.

13. Method according to claim 11, the method comprising at least one of the following partial steps: B1: applying at least one, several, or all layers of said ceramic insulation onto said stack arrangement by at least one of coating, coating with a green tape, injection molding, plasma spraying, immersion coating, coating in ceramic slurry, spraying, and/or by way of a sol-gel method; B2: sintering said ceramic insulation and/or said ceramic basis material so that said ceramic insulation and/or said ceramic basis material is/are transformed into a solid ceramic structure; and/or B3: setting an average grain size of said ceramic insulation and/or an average grain size of said ceramic basis material by selecting different materials for said ceramic basis material and said ceramic insulation and/or by selecting process parameters during sintering.

14. Method according to claim 11, comprising: C: polarizing said ceramic basis material and/or said ceramic insulation to set the electromechanical properties of said actuator.

15. Method according to claim 11, where said stack arrangement in step A is formed from electrodes and ceramic basis material along a stack axis so that said stack arrangement includes two end faces oriented perpendicular to said stack axis and at least one side surface extending between said end faces, and said stack arrangement in step B is provided with said ceramic insulation such that the latter extends on said at least one side surface from one end face to the other end face.

16. Actuator according to claim 2, wherein an average grain size of said ceramic insulation structure is selected to be one or more of 20 μm or less, 5 μm or less, and/or 1 μm or less.

17. Actuator according to claim 16, wherein an average grain size of the ceramic basis material decreases continuously towards said ceramic insulation, and/or beyond said ceramic insulation.

18. Method according to claim 12, the method comprising at least one of the following partial steps: B1: applying at least one, several, or all layers of said ceramic insulation onto said stack arrangement by at least one of coating, coating with a green tape, injection molding, plasma spraying, immersion coating, coating in ceramic slurry, spraying, and/or by way of a sol-gel method; B2: sintering said ceramic insulation and/or said ceramic basis material so that said ceramic insulation and/or said ceramic basis material is/are transformed into a solid ceramic structure; and/or B3: setting an average grain size of said ceramic insulation and/or an average grain size of said ceramic basis material by selecting different materials for said ceramic basis material and said ceramic insulation and/or by selecting process parameters during sintering.

19. Method according to claim 18, comprising: C: polarizing said ceramic basis material and/or said ceramic insulation to set the electromechanical properties of said actuator.

20. Method according to claim 19, where said stack arrangement in step A is formed from electrodes and ceramic basis material along a stack axis so that said stack arrangement includes two end faces oriented perpendicular to said stack axis and at least one side surface extending between said end faces, and said stack arrangement in step B is provided with said ceramic insulation such that the latter extends on said at least one side surface from one end face to the other end face.

Description

BRIEF DESCRIPTION OF THE FIGURES

[0038] FIG. 1 shows a top view of the actuator according to the invention as well as sections along the lines A-A and B-B.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0039] FIG. 1 shows an electromechanical actuator consisting of a rod-shaped stack arrangement 1 made of ceramic basis material having electromechanical properties and electrodes 2 which are alternately led on opposite side surfaces of the actuator. An exterior electrode 3 for contacting electrodes 2 is applied to each of these two side surfaces. External electrodes 3 are each connected to a connecting lead 4. The side surfaces of the actuator that are not covered by the external electrodes are provided with ceramic insulation 5.

[0040] The basic structure of this stack arrangement is known from DE 100 21 919 C2 and is not explained in more detail at this point. Instead, the differences according to the invention from the known stack arrangement, which shall be explained hereafter, shall be discussed primarily.

[0041] According to the invention, the structure of ceramic insulation 5 has a smaller average grain size than the structure of the ceramic basis material. Due to the small average grain size, a small pore size or porosity arises in the structure of ceramic insulation 5, as a result of which the defect size in the structure decreases and counteracts the formation or growth of cracks.

[0042] The ceramic basis material and the electrodes are preferably arranged along a stack axis, where the stack arrangement composed of ceramic basis material and electrodes has two end faces aligned perpendicular to the stack axis and at least one side surface extending between the end faces, and the ceramic insulation on the at least one side surface extends from one end face to the other end face. Without the ceramic insulation, the electrodes would be exposed on the circumferential side surfaces. Providing the ceramic insulation on these side surfaces prevents the diffusion or penetration of water vapor molecules between the electrodes and the ceramic basis material.

[0043] In a preferred embodiment, the ratio of the average grain sizes of insulation to ceramic basis material is ¾ or less, preferably ½ or less, particularly preferably ⅓ or less. Where the average grain size of the insulation can be 20 μm or less, preferably 5 μm or less, particularly preferably 1 μm or less.

[0044] The ceramic basis material is preferably piezoceramic material, such as e.g. lead zirconate titanate (PZT). In addition, however, it can also be electrostrictive or magnetostrictive material. Ceramic insulation 5 is likewise made of PZT or oxide ceramic material and preferably likewise has piezoelectric properties.

[0045] Ceramic insulation 5 can be composed of a single layer. In a preferred embodiment, however, ceramic insulation 5 has several layers, in particular two or three, in order to cover defects caused by manufacturing technology in one single layer with another layer. This can prevent the growth of cracks at the boundary between two insulation layers. As a result, ceramic insulation 5 can be made to be reliably impermeable to the penetration or diffusion of water vapor molecules into the stack arrangement. The individual layers can differ from one another in their composition or in their average grain size. The thickness of an individual layer is a multiple, preferably five to twenty times, the average grain size in this layer. In general, a small overall thickness of the ceramic insulation is to be aimed for, since the expansion of the ceramic basis material is less restricted by a thin insulation. In a preferred embodiment, the overall thickness of the insulation is 500 μm or less, preferably 100 μm or less, particularly preferably 60 μm or less.

[0046] A method according to the invention for the production of an actuator, preferably an actuator according to the above embodiments, shall be described hereafter.

[0047] The production of a stack arrangement from a ceramic basis material having electromechanical properties and electrodes is known in principle from DE 100 21 919 C2, but shall be outlined again briefly to improve the understanding of the method according to the invention.

[0048] The starting point for the production of stack arrangement 1 is ceramic powder material, preferably PZT powder material which is provided with a binder solution and a solvent. In the further course of the method, the ceramic slurry made of powder material and binder solution is poured to form so-called green tapes made of ceramic basis material, and the solvent evaporates. The flexible green tapes are then cut to size and stacked, where an electrode 2 is inserted between each two layers of a predefined number of green tapes. For this purpose, a metal paste is printed onto the respective green tape using a screen-printing method. Stack arrangement 1 produced in this manner is then isostatically pressed.

[0049] The method according to the invention now provides for such a stack arrangement 1 to be provided with ceramic insulation 5 so that the structure of ceramic insulation 5 has a smaller average grain size than the structure of the ceramic basis material. For this purpose, one or more green tapes with different material composition or with different material properties than the green tapes of the ceramic basis material are applied in a preferred embodiment to at least one side surface of stack arrangement 1. Stack arrangement 1 and ceramic insulation 5 are subsequently jointly sintered, as a result of which the green tapes of the ceramic basis material and ceramic insulation 5 are transformed into a solid ceramic structure. By selectively setting the compositions and/or the properties of the starting materials of the ceramic basis material and ceramic insulation 5, a smaller average grain size can be obtained in the solid structure of ceramic insulation 5 than in the structure of the ceramic basis material. After sintering, external electrodes 3 are applied to the side surfaces of the actuator and are configured to establish electrical contact with electrodes 2 in stack arrangement 1 and have no ceramic insulation 5. After the external electrodes have been applied, a unidirectional electric field is applied to create a polarization for polarizing the ceramic basis material and preferably ceramic insulation 5.

[0050] The method described above is a single-stage process since stack arrangement 1 and ceramic insulation 5 are sintered together. In addition, it is also possible to carry out a two-stage process in which stack arrangement 1 is first sintered, then one or more layers of ceramic insulation 5 is/are applied onto sintered stack arrangement 1, and the stack arrangement provided with ceramic insulation 5 is sintered again. The different average grain sizes in the structure of ceramic insulation 5 and the ceramic basis material can there be set in particular by the selective selection of the process parameters in the individual sintering stages, where the composition or properties of the starting materials of the ceramic basis material and ceramic insulation 5 can be substantially the same.

[0051] According to the above embodiments, the average grain sizes in the respective microstructure in the single-stage process is set substantially by way of the different compositions or properties of the starting materials of the ceramic basis material and ceramic insulation 5, in the two-stage process, however, substantially by the selection of the process parameters in the individual sintering processes. However, the method according to the invention is not restricted to such a single-stage or multi-stage method.

[0052] In addition, the desired average grain size in the respective microstructure can also be set in the single-stage process by the suitable selection of the process parameters. Microwave sintering, which shall be described in more detail hereafter, results in non-uniform heat distribution in the stack arrangement in which lower temperatures arise in the edge regions of the insulation due to the energy radiated via the surface of the stack arrangement, so that the grain growth at the center is greater than in the insulation.

[0053] Non-uniform temperature distribution in the stack arrangement during the sintering process is also possible by applying a suitable electrical voltage to the actuators since the resulting current flow causes heating, and, due to of the energy irradiation at the surface of the stack arrangement, it has a lower temperature than its core region. The electrical field in the interior of the stack arrangement resulting from the electrical voltage can additionally have a positive influence on grain growth or the grain size distribution in the stack and the insulation.

[0054] In addition, the grain growth can also be influenced by an external electric field in the above-mentioned sense (gradual progression of the grain size in the ceramic insulation) without there being any conductive contact to the actuators.

[0055] Furthermore, different material compositions or material properties can also be used in the two-stage process in addition to different process parameters in the individual sintering processes.

[0056] Some examples for the realization of different compositions or properties of the starting materials which can contribute to setting different average grain sizes in the respective structures shall be explained hereafter.

[0057] Firstly, it is advisable to use a starting material for the ceramic basis material, the average grain size of which with typical sintering is sufficiently large to achieve expansions >1.1‰. For ceramic insulation 5, this material can be doped with a grain growth inhibitor in order to inhibit the grain growth in the structure of ceramic insulation 5 as compared to the grain growth in the structure of the ceramic basis material in a subsequent sintering process.

[0058] Secondly, a starting material can be used for ceramic insulation 5, the average grain size of which, with typical sintering, is sufficiently small to meet the strength requirements for the insulation. For the ceramic basis material, this material can be doped with a grain growth accelerator in order to accelerate the grain growth in the structure of ceramic basis material as compared to the grain growth in the structure of ceramic insulation 5 in a subsequent sintering process.

[0059] Furthermore, the starting materials of the ceramic basis material and ceramic insulation 5 can differ in their initial grain size. This means that a particularly finely ground powder can be used for ceramic insulation 5 as compared to the ceramic basis material, which leads to a smaller average grain size in the structure of ceramic insulation 5 even after sintering.

[0060] In addition, especially in the case of PZT materials, there is the possibility of selecting starting materials for the ceramic basis material and ceramic insulation 5 that differ in their affinity for lead. If a starting material with a higher affinity for lead than the starting material for ceramic insulation 5 is selected for the ceramic basis material, then the ceramic basis material removes part of the lead that is contained from the ceramic insulation. This removal slows the grain growth dynamics in ceramic insulation 5.

[0061] In order to set the desired difference in the average grain sizes in the respective microstructures, the above-mentioned options for realizing different material compositions or material properties, respectively, can be used individually or in combination.

[0062] The temperature or the temperature profile over time, the holding time, electrical fields in the environment, and the ambient atmosphere, in particular the oxygen content and the atmospheric pressure, substantially describe the process parameters that can be set for the respective sintering processes which individually or in combination can contribute to realizing different average grain sizes in the respective microstructure.

[0063] In addition, the difference in the average grain size in the respective structures can also be obtained through the use of microwave sintering. Due to the dipole structure of piezoceramics, the heat develops in the volume of the component during microwave sintering. Since heat is dissipated from the surface of the component to the colder environment, i.e. the atmosphere and walls of the sintering assembly, the sintering temperature of the surface and therefore of ceramic insulation 5 is below the core temperature of the ceramic basis material. As a result of this temperature difference, the grain growth in ceramic insulation 5 proceeds more slowly.

[0064] The present method is not restricted to the application of layers of ceramic insulation 5 in the form of a green tape. There are also the possibilities of applying the starting material of ceramic insulation 5 by injection molding, plasma spraying, immersion coating in ceramic slurry, spraying or in by way of a sol-gel method. With the exception of plasma spraying, all of the above-mentioned methods of layer application can be used for both a single-stage as well as a two-stage sintering process, as described above. Plasma-sprayed layers do not have to be sintered again, but can already have their desired properties after the layer has been applied. Subsequent temperature treatment, however, can be advantageous.

[0065] Each of the above-mentioned methods of layer application can be combined with the options already mentioned for the different material compositions or material properties and the process parameters during sintering. Furthermore, one or more layers of ceramic insulation 5 can be realized with all of the methods mentioned.

[0066] A grain size gradient can be set via the ceramic insulation 5 by selecting different material compositions or material properties in each layer of ceramic insulation 5 or by individually sintering each of these layers. Furthermore, as a result of interaction, in particular diffusion, at the layer boundaries of layers with different material compositions or material properties, a grain size gradient arises at these layer boundaries. With such a grain size gradient, expansion or tension cracks are prevented.

[0067] Already polarized actuators according to the invention were subjected to a static life test at a constant voltage (DC). At the same time, prior art actuators with ceramic insulation were investigated. The control voltage was selected such that the actuators showed an expansion of the active region of 1.47‰. The mean service life MTTF of the actuators was determined based on the times to failure of the actuators for each test group. The values determined and the test conditions can be gathered from Table 1.

[0068] The MTTF for polymer-coated actuators was calculated using the service life formula of the respective manufacturer.

[0069] The MTTF of the actuators according to the invention at 25° C. and 30% relative humidity was determined by extrapolation based on a series of tests.

TABLE-US-00001 TABLE 1 mean service life type of actuator test conditions MTTF polymer-coated multi-layer 1.47‰ expansion, 80° C., 44 h actuators 80% relative humidity, DC ceramic insulated multi-layer 1.47‰ expansion, 80° C., 1360 h actuators having the same 80% relative humidity, DC average grain size in the insulation as in the basis material ceramic insulated multi-layer 1.47‰ expansion, 80° C., 2460 h actuators having a smaller 80% relative humidity, DC average grain size in the insulation than in the basis material (according to the invention) ceramic insulated multi-layer 1.47‰ expansion, 25° C., >500000 h actuators having a smaller 30% relative humidity, DC average grain size in the insulation than in the basis material (according to the invention)

LIST OF REFERENCE CHARACTERS

[0070] 1 stack arrangement [0071] 2 electrodes [0072] 3 external electrode [0073] 4 connecting lead [0074] 5 ceramic insulation