Electromechanical actuator with ceramic insulating layer and method for manufacturing the same

Abstract

This invention relates to an electromechanical actuator comprising a stacked assembly (1) consisting of a ceramic base material having electromechanical properties and electrodes (2), and a ceramic insulating layer (5) for use in humid environments. To ensure a long service life of the actuator under conditions of increased electromechanical expansion, according to the invention, the ceramic insulating layer (5) has a smaller average grain size than the ceramic base material. Another aspect of the invention relates to a method for manufacturing an actuator having a ceramic insulating layer and a method for operating such an actuator.

Description

Technical Field

[0001] This invention relates to an electromechanical actuator according to the preamble of claim 1, comprising a stacked assembly consisting of a ceramic base material having electromechanical properties and electrodes, and a ceramic insulator for use in humid environments. Such an actuator is also known as a multilayer actuator or a linear actuator. Another aspect of the invention relates to a method for manufacturing an actuator having a ceramic insulating layer and a method for operating such an actuator. Background Technology

[0002] The lifespan of electromechanical actuators is primarily limited by degradation mechanisms caused by air humidity, due to their use in static or quasi-static applications. In particular, the diffusion or intrusion of water vapor molecules into the stacked assembly causes partial or complete loss of actuator function, resulting in degradation of the actuator's electrical and elastic properties. Improved lifespan can be achieved through the actuator's insulation layer, particularly insulation layers made of ceramic materials. Such an electromechanical or piezoelectric actuator with a ceramic insulation layer is known from DE 100 21 919C2. Compared to actuators with polymer insulation layers, ceramic-insulated actuators can have a mean time to failure (MTTF) that is two to three orders of magnitude higher when operated with direct current (DC) voltage.

[0003] A piezoelectric actuator is also known from EP 2 530 756 A1, which has an inorganic coating, for example, made of lead zirconate titanate (PZT), on its side surface. US 7,065,846 B2 also discloses a piezoelectric ceramic actuator with a ceramic insulating layer.

[0004] However, such ceramic insulation layers have physical limitations. To date, actuators with ceramic coatings, when operated using an electric field strength of 2 kV-DC / mm, typically exhibit inductively induced piezoelectric expansion of approximately 1.0‰ to 1.1‰ of the effective length of the actuator. Increasing this inductively induced piezoelectric expansion to, for example, >1.25‰ of the effective length, as can be achieved, for example, through optimization of the electromechanical material system lead zirconate titanate (PZT), leads to uncontrolled crack formation and propagation in the ceramic insulation layer, adversely affecting the actuator's lifespan. With linear actuator designs, the electrodes do not extend into the insulation layer because the electrodes require moisture protection. Therefore, the ceramic insulation layer may only be excited by stray electric fields, naturally resulting in lower inductively induced piezoelectric expansion, or even no expansion at all. For this reason, mechanical tensile stress is generated during actuator operation or expansion, which can lead to crack formation due to continuous or alternating loads and may cause actuator failure or malfunction, or shorten its lifespan in humid environments.

[0005] WO 2017 / 032868 A1 discloses a piezoelectric multilayer actuator having a protective layer formed on the actuator surface by airflow separation (see page 4, lines 14-16). This protective layer consists of pulverized ceramic particles bonded together, for example, by annealing (see page 4, lines 28-5, lines 2). This protective layer is to be very dense and non-porous and free of "grain boundaries" such as those generated by a sintering process, so that the protective layer is claimed to remain crack-free during actuator operation even with a small thickness (see page 5, lines 5-10). Summary of the Invention

[0006] EP 2 667 425 B1 discloses a piezoelectric element having an inorganic coating in which metal particles are dispersed. Due to the metal particles, there is a risk of conductive paths forming within the coating, particularly at defects, which could potentially accelerate the failure mechanism. The object of this invention is to provide an electromechanical actuator that, even with increased electromechanical expansion, has an extended service life by preventing crack formation in the sintered ceramic insulating layer.

[0007] This object of the invention is achieved by the electromechanical actuator according to claim 1, the method for manufacturing the electromechanical actuator according to claim 10, and the method for operating the electromechanical actuator according to claim 14. Advantageous modifications of the invention are claimed in the dependent claims.

[0008] The electromechanical actuator according to the invention comprises a stacked assembly consisting of a ceramic base material having electromechanical properties and electrodes, and a ceramic insulating layer for use in humid environments, wherein the microstructure of the ceramic insulating layer has a smaller average grain size than that of the ceramic base material. By specifically selecting the ceramic insulating layer (which has a smaller average grain size than that of the ceramic base material), the strength (yield limit, see Hall-Page formula) of the ceramic insulating layer and its resistance to crack formation are improved. By improving this resistance, an extended service life of the actuator can be achieved even in cases of significant electromechanical expansion, which is achievable due to the larger average grain size in the microstructure of the ceramic base material, as the available expansion caused by the electroinduction of the material increases with increasing grain size. Preferably, the ceramic base material is a piezoelectric ceramic base material. Preferably, the electromechanical properties of the actuator are modified or adjusted, for example, by polarization, after the ceramic base material is formed.

[0009] It is advantageous for the ratio of the average particle size of the insulating layer to the average particle size of the ceramic base material to be 3 / 4 or less, preferably 1 / 2 or less, and particularly preferably 1 / 3 or less. In principle, the smaller the average particle size of the insulating layer, the higher its strength and resistance to crack growth. This ratio achieves favorable results while taking into account the mechanical interactions between the base material and the insulating layer.

[0010] Furthermore, it would be suitable for the insulating layer to have an average particle size of 20 μm or less, preferably 5 μm or less, and particularly preferably 1 μm or less.

[0011] Another proven practical feature is that the average particle size of the ceramic base material decreases towards the insulating layer, preferably beyond the insulating layer, and preferably continuously. By continuously reducing the average particle size in the region where the insulating layer and the ceramic base material meet, abrupt changes in expansion or stress in this region can be avoided.

[0012] It can be proven to be beneficial that the insulating layer is made of electromechanical materials, preferably piezoelectric materials, and particularly preferably piezoelectric ceramic materials.

[0013] Furthermore, it is beneficial that the insulation layer is impermeable to water vapor / moisture. This prevents water vapor molecules from diffusing or penetrating into the stacked components.

[0014] It would be useful if the insulating layer consisted of at least one layer, preferably two or three layers, wherein the total thickness of the insulating layer was 500 μm or less, preferably 100 μm or less, and particularly preferably 60 μm or less. By providing multiple layers, processing defects in a single layer, such as cavities or pores, can be covered by additional layers, resulting in improved insulation. Thus, the probability of defects penetrating the entire thickness of the insulating layer is reduced by each additional layer.

[0015] Furthermore, it would be beneficial that the insulating layer has essentially the same or different material composition as the ceramic base material.

[0016] Furthermore, it is advantageous to provide two external electrodes for communication with electrode contacts in the stacked assembly. These external electrodes are disposed on the same or two different outer surfaces of the actuator, wherein at least one electrodeless outer surface of the actuator has a ceramic insulating layer. By mounting the external electrodes and providing a ceramic insulating layer on the electrodeless outer surface, the actuator can be reliably insulated.

[0017] Ideally, the ceramic base material and electrodes should be arranged along a stacking axis, and the stack assembly should have two end faces oriented perpendicular to the stacking axis and at least one side surface extending between these end faces, wherein a ceramic insulating layer extends from one end face to the other on the at least one side surface. Providing a ceramic insulating layer on these side surfaces prevents water vapor molecules from diffusing or penetrating between the electrodes and the ceramic base material.

[0018] Another aspect of the present invention relates to a method for manufacturing an electromechanical actuator, particularly an actuator according to one of the aforementioned embodiments, comprising the steps of:

[0019] A: The stacked assembly consists of electrodes and a ceramic base material with electromechanical properties.

[0020] B: A ceramic insulating layer is provided for the stacked assembly such that the microstructure of the insulating layer has a smaller average grain size than that of the ceramic base material.

[0021] Furthermore, it would be beneficial that the method includes at least one of the following sub-steps:

[0022] A1: A stacked assembly consisting of film preforms made of electrodes and ceramic base materials.

[0023] A2: Sintering the stacked components transforms the ceramic base material into a solid-phase ceramic structure.

[0024] However, it can also be beneficial that the method includes at least one of the following sub-steps:

[0025] B1: Applying at least one, multiple, or all layers of the insulating layer to the stacked assembly by coating, preferably coating a preform, injection molding, plasma spraying, impregnation encapsulation, preferably impregnation encapsulation in a ceramic slurry, spraying, and / or using a sol-gel method.

[0026] B2: Sintering the ceramic insulating layer and, if necessary, the ceramic base material, transforming the ceramic insulating layer and, if necessary, the ceramic base material into a solid-phase ceramic structure.

[0027] B3: Adjust the average particle size of the insulation layer and, if necessary, the average particle size of the ceramic base material by selecting different materials for the ceramic base material and the ceramic insulation layer and / or by selecting process parameters during sintering.

[0028] Another advantage is that the method includes the following steps:

[0029] C: Polarize the ceramic base material, and preferably polarize the ceramic insulating layer, in order to adjust the electromechanical characteristics of the actuator.

[0030] Furthermore, it would be suitable to: in step A, construct a stacked assembly of electrodes and ceramic base material along a stacking axis, such that the stacked assembly has two end faces oriented perpendicular to the stacking axis and at least one side surface extending between these end faces; and in step B, provide a ceramic insulating layer for the stacked assembly such that the ceramic insulating layer extends from one end face to the other on the at least one side surface. As explained above, providing a ceramic insulating layer on these side surfaces prevents water vapor molecules from diffusing or penetrating between the electrodes and the ceramic base material.

[0031] Another aspect of the invention relates to a method for operating an electromechanical actuator, particularly an actuator according to any of the foregoing embodiments, the actuator having an electric field strength of at least 2 kV-DC / mm at room temperature and preferably at an air humidity of at least 80%, preferably at least 85%, preferably at least 90%, and particularly preferably at least 92%, thereby achieving a quasi-static expansion of 1.25‰ or more of the effective length of the actuator and a mean time to use (MTTF) of 10,000 hours or more for the actuator.

[0032] Another aspect of the invention relates to a method for operating an electromechanical actuator, particularly an actuator according to any of the foregoing embodiments, the actuator having an electric field strength of at least 2 kV-DC / mm at 90°C and preferably at at least 80%, preferably at least 85%, preferably at least 90%, and particularly preferably at least 92% air humidity, thereby achieving a quasi-static expansion of 1.25‰ or more of the effective length of the actuator and a mean time to use (MTTF) of 500 hours or longer for the actuator.

[0033] Terms and Definitions

[0034] Film preform made of ceramic material

[0035] A flexible membrane made from ceramic slurry through a membrane casting process is called a membrane preform made of ceramic material.

[0036] Insulation layer

[0037] The protective layer that prevents moisture from damaging the stacked actuator is called the insulation layer.

[0038] Actuator

[0039] The term "executor" is used as a synonym for stacked actuator, multi-layered actuator, multi-layered actuator (Mehrschichtaktor), or linear actuator.

[0040] Ceramic base materials with electromechanical properties

[0041] Materials that undergo mechanical (elastic) deformation when a voltage is applied are called ceramic-based materials with electromechanical properties. The starting point for such materials can be ceramic powder, which is then sintered. Sintered ceramics are polycrystalline materials in their microstructure, with crystallites possessing dipole-bound regions whose orientation is statistically distributed throughout the material. To adjust the electromechanical properties of the actuator, the dipoles are rectified, preferably through polarization.

[0042] polarization

[0043] To manufacture most electromechanical actuators, the material is polarized after sintering by applying an external DC electric field; that is, all dipoles are rectified. This polarization allows for the adjustment of the desired electromechanical characteristics of the actuator. Attached Figure Description

[0044] Figure 1 A top view of the actuator according to the invention and a cross-sectional view along lines AA and BB are shown. Detailed Implementation

[0045] Figure 1 An electromechanical actuator is shown, comprising a strip-shaped stacked assembly 1 composed of a ceramic base material with electromechanical properties and electrodes, which are alternately guided on opposing side surfaces of the actuator. An external electrode 3 for contacting electrodes 2 is provided on each of the two side surfaces. The external electrode 3 is connected to a connecting conductor 4. A ceramic insulating layer 5 is provided on the side surface of the actuator not covered by the external electrodes.

[0046] The basic structure of such a stacked assembly is known from DE 100 21 919 C2 and will not be described in detail here. Instead, this paper mainly discusses the differences between the present invention and the known stacked assembly, which will be explained below.

[0047] According to the present invention, the ceramic insulating layer 5 has a smaller average grain size than the ceramic base material. Due to the small average grain size, small pore size or porosity is generated in the structure of the ceramic insulating layer 5, thereby reducing the size of defects in the structure and preventing the formation or growth of cracks.

[0048] Preferably, the ceramic base material and the electrode are arranged along a stacking axis, wherein the stacked assembly including the ceramic base material and the electrode has two end faces oriented perpendicular to the stacking axis and at least one side surface extending between these end faces, and a ceramic insulating layer extends from one end face to the other on the at least one side surface. Without the ceramic insulating layer, the electrode would be exposed on the peripheral side surfaces. Therefore, providing a ceramic insulating layer on these side surfaces prevents the diffusion or intrusion of water vapor molecules between the electrode and the ceramic base material.

[0049] In a preferred embodiment, the ratio of the average particle size of the insulating layer to the average particle size of the ceramic base material is 3 / 4 or less, preferably 1 / 2 or less, and particularly preferably 1 / 3 or less. Here, the average particle size of the insulating layer can be 20 μm or less, preferably 5 μm or less, and particularly preferably 1 μm or less.

[0050] The ceramic base material is preferably a piezoelectric ceramic material, such as lead zirconate titanate (PZT). However, it can also be an electrostrictive or magnetostrictive material. The ceramic insulating layer 5 is also composed of PZT or oxide ceramic and preferably also has piezoelectric properties.

[0051] The ceramic insulating layer 5 can consist of a single layer. However, in a preferred embodiment, the ceramic insulating layer 5 has multiple layers, particularly two or three layers, to cover defects in a single layer caused by the processing with another layer. This prevents crack growth at the boundary between the two insulating layers. Therefore, the ceramic insulating layer 5 can be made truly impermeable to the intrusion or diffusion of water vapor molecules into the stacked assembly. The individual layers can differ from each other in their composition or average particle size. The thickness of a single layer is here many times, preferably five to twenty times, the average particle size in that layer. Generally, a small total thickness of the ceramic insulating layer is preferred because a thin insulating layer restricts the expansion of the ceramic base material less. In a preferred embodiment, the total thickness of the insulating layer is 500 μm or less, preferably 100 μm or less, and particularly preferably 60 μm or less.

[0052] The following describes a method for manufacturing an actuator according to the present invention, preferably an actuator according to the above-described embodiment.

[0053] It is known in principle, by DE 100 21 919 C2, to manufacture stacked assemblies from ceramic base materials and electrodes with electromechanical properties; however, a brief overview is given again for better understanding of the method according to the invention.

[0054] Ceramic powder material, preferably PZT powder material, is the starting point for manufacturing the stacked assembly 1. This powder material is formulated with a binder solution and a solvent. In a further process, a ceramic slurry consisting of the powder material and the binder solution is cast into a so-called preform made of a ceramic base material, and the solvent evaporates. The flexible preforms are then cut to length and stacked, wherein an electrode 2 is placed between each pair of layers consisting of a predetermined number of preforms. For this purpose, a metal paste is printed onto the corresponding preforms using screen printing. The stacked assembly 1 thus manufactured is then subjected to isobaric extrusion.

[0055] The method according to the invention now specifies that a ceramic insulating layer 5 is provided for such a stacked assembly 1, such that the microstructure of the ceramic insulating layer 5 has a smaller average grain size than that of the ceramic base material. For this purpose, in a preferred embodiment, one or more film preforms having different material compositions or properties than the film preforms of the ceramic base material are laid onto at least one side surface of the stacked assembly 1. The stacked assembly 1 and the ceramic insulating layer 5 are then sintered together, thereby transforming the film preforms of the ceramic base material and the ceramic insulating layer 5 into a solid-phase ceramic microstructure. By selectively adjusting the composition and / or properties of the raw materials of the ceramic base material and the raw materials of the ceramic insulating layer 5, a smaller average grain size can be achieved in the solid-phase microstructure of the ceramic insulating layer 5 than in the microstructure of the ceramic base material. After sintering, external electrodes 3 are laid onto the side surface of the actuator; these external electrodes are configured for electrical contact with the electrodes 2 in the stacked assembly 1 and do not have a ceramic insulating layer 5. After the external electrodes are laid, a DC electric field is applied to polarize the ceramic base material and, preferably, the ceramic insulating layer 5.

[0056] Because the stacked assembly 1 and the ceramic insulating layer 5 are sintered together, the above method is a one-step method. Alternatively, a two-step method can be implemented, in which the stacked assembly 1 is first sintered, then one or more ceramic insulating layers 5 are applied to the sintered stacked assembly 1, and then the stacked assembly with the ceramic insulating layers 5 is sintered again. In this case, in particular, different average particle sizes in the microstructure of the ceramic insulating layer 5 and the microstructure of the ceramic base material can be adjusted by selectively choosing process parameters in each sintering stage, wherein the raw materials of the ceramic base material and the raw materials of the ceramic insulating layer 5 can have substantially the same composition or properties.

[0057] According to the above embodiments, in the one-step method, the average particle size in the corresponding microstructure is adjusted primarily by the different compositions or properties of the raw materials of the ceramic base material and the raw materials of the ceramic insulating layer 5, while in the two-step method, it is adjusted primarily by the selection of process parameters in each sintering process. However, the method according to the present invention is not limited to such one-step or multi-step methods.

[0058] Furthermore, in the one-step method, the desired adjustment of the average grain size in the corresponding microstructure can also be achieved by appropriately selecting process parameters. In the case of microwave sintering, which is described in more detail below, a non-uniform heat distribution is generated in the stacked assembly, wherein a lower temperature exists in the edge region of the insulating layer due to the energy radiated to the surface of the stacked assembly, resulting in greater grain growth in the center than in the insulating layer.

[0059] Non-uniform temperature distribution within the stacked assembly during the sintering process can also be achieved by applying an appropriate voltage to the actuator. This is because the generated current acts as a heater, and the surface of the stacked assembly has a lower temperature than the core region due to energy radiation. Furthermore, the electric field generated by the voltage within the stacked assembly can positively influence grain growth or grain size distribution within the stack and the insulating layer.

[0060] Furthermore, even without conductive contact with the actuator, grain growth can still be affected by the external electric field in the sense described above (the gradual process of grain size development in ceramic insulation).

[0061] In addition, in the two-step method, different material compositions or material properties can be used, in addition to the different process parameters in each sintering process.

[0062] The following examples illustrate different compositions or properties of the substrate, which can help to adjust different average particle sizes in the corresponding structures.

[0063] First, a raw material with an average particle size large enough to achieve an expansion of >1.1‰ in typical sintering is recommended for the ceramic base material. For the ceramic insulating layer 5, a grain growth inhibitor can be added to this material to suppress grain growth in the microstructure of the ceramic insulating layer 5 relative to the grain growth in the microstructure of the ceramic base material during the subsequent sintering process.

[0064] Secondly, the following raw materials can be used for the ceramic insulating layer 5, whose average particle size is small enough in typical sintering to meet the strength requirements of the insulating layer. For the ceramic base material, a grain growth accelerator can be added to this material to accelerate grain growth in the microstructure of the ceramic base material relative to the grain growth in the microstructure of the ceramic insulating layer 5 during the subsequent sintering process.

[0065] Furthermore, the raw materials for the ceramic base material and the raw materials for the ceramic insulating layer 5 can differ in their initial particle size. That is, compared to the ceramic base material, a particularly finely ground powder can be used for the ceramic insulating layer 5, which also results in a smaller average particle size in the microstructure of the sintered ceramic insulating layer 5.

[0066] Furthermore, particularly for PZT materials, it is possible to select raw materials with different lead affinities for the ceramic base material and the ceramic insulating layer 5. If a raw material with a higher lead affinity is selected for the ceramic base material than that used for the ceramic insulating layer 5, then the ceramic base material extracts a portion of the lead contained in the ceramic insulating layer. This extraction slows down the grain growth dynamics in the ceramic insulating layer 5.

[0067] In order to adjust for the desired difference in average particle size in the corresponding organization, the above possibilities can be used individually or in combination to achieve different material compositions or material properties.

[0068] For the corresponding sintering process, temperature or temperature-time curves, holding time, electric field and atmospheric environment, especially oxygen content and atmospheric pressure, essentially describe the adjustable process parameters that can individually or in combination contribute to achieving different average particle sizes in the corresponding microstructure.

[0069] Alternatively, different average grain sizes within the corresponding microstructure can be achieved using microwave sintering. During microwave sintering, heat is generated within the volume of the component due to the dipole structure of the piezoelectric ceramic. Because this heat is released from the component surface into a cooler environment—that is, into the atmosphere and the walls of the sintering facility—the sintering temperature of the surface, and consequently the sintering temperature of the ceramic insulating layer 5, is lower than the core temperature of the ceramic base material. This temperature difference results in slower grain growth in the ceramic insulating layer 5.

[0070] This method is not limited to the layer coating of ceramic insulating layer 5 in the form of a preform. Other possibilities include applying the raw material of ceramic insulating layer 5 via injection molding, plasma spraying, impregnation in ceramic slurry, spraying, or sol-gel methods. Except for plasma spraying, all the above-mentioned layer coating methods can be used for both one-step and two-step sintering processes, as described above. Plasma-sprayed layers do not need to be sintered again and already possess their desired properties after layer coating. However, subsequent temperature treatment would be advantageous.

[0071] Each of the above-described layer coating methods can be combined with the listed possibilities of different material compositions or material properties and the listed possibilities of process parameters during sintering. Furthermore, one or more ceramic insulating layers 5 can be achieved using all the listed methods.

[0072] The grain size gradient on the ceramic insulating layer 5 can be adjusted by selecting different material compositions or properties in each layer, or by sintering each layer individually. Furthermore, the grain size gradient at the layer boundaries is caused by the interaction, particularly diffusion, between layers with different material compositions or properties. Such a grain size gradient helps to avoid abrupt changes in expansion or stress.

[0073] The polarized actuators according to the present invention underwent static life testing using a constant voltage (DC). Simultaneously, actuators with ceramic insulating layers according to the prior art were also investigated. The control voltage was selected such that the effective area of ​​the actuator expanded by 1.47‰. The mean time factor (MTTF) of the actuators was calculated based on the failure time points of the actuators in each test group. The calculated values ​​and test conditions can be obtained from Table 1.

[0074] The MTTF of polymer-encapsulated actuators is calculated using the lifespan formula provided by the respective manufacturer.

[0075] The MTTF of the actuator according to the present invention at 25°C and 30% relative humidity was determined by extrapolation based on a series of tests.

[0076] Table 1

[0077]

[0078] List of reference numerals

[0079] 1 Stacked Components

[0080] 2 electrodes

[0081] 3 External Electrode

[0082] 4 Connecting conductors

[0083] 5. Ceramic insulation layer

Claims

1. An electromechanical actuator comprising a stacked assembly (1) consisting of a ceramic base material having electromechanical properties and electrodes (2) and a ceramic insulating layer (5) for use of the actuator in a humid environment, wherein, The ceramic base material and the electrode (2) are arranged along the stacking axis, and the stacking assembly (1) has two end faces oriented perpendicular to the stacking axis and at least one side surface extending between these end faces, wherein the ceramic insulating layer (5) has a smaller average grain size than the ceramic base material, characterized in that: the ceramic insulating layer (5) extends from one end face to the other on the at least one side surface, and the ceramic insulating layer (5) has a different material composition than the ceramic base material, wherein the material of the ceramic insulating layer (5) is doped with grain growth inhibitors, and in the region where the ceramic insulating layer (5) and the ceramic base material meet, the average grain size of the ceramic base material decreases toward the ceramic insulating layer (5).

2. The actuator according to claim 1, characterized in that: The ratio of the average particle size of the insulating layer (5) to the average particle size of the ceramic base material is 3 / 4 or less.

3. The actuator according to claim 1 or 2, characterized in that: The average particle size of the insulating layer (5) is 20 μm or smaller.

4. The actuator according to claim 1 or 2, characterized in that: The average particle size of the ceramic base material decreases continuously toward the insulating layer (5).

5. The actuator according to claim 1 or 2, characterized in that: The insulating layer (5) is made of electromechanical materials.

6. The actuator according to claim 1 or 2, characterized in that: The insulating layer (5) is impermeable to water vapor / moisture.

7. The actuator according to claim 1, characterized in that: The insulating layer (5) consists of at least one layer.

8. The actuator according to claim 1 or 2, characterized in that: Two external electrodes (3) are provided for contacting the electrodes (2) in the stacked assembly (1). These external electrodes are provided on the same external surface or two different external surfaces of the actuator, wherein the external surface of at least one external electrode of the actuator has the ceramic insulating layer (5).

9. The actuator according to claim 1, characterized in that: The ratio of the average particle size of the insulating layer (5) to the average particle size of the ceramic base material is 1 / 2 or less.

10. The actuator according to claim 1, characterized in that: The ratio of the average particle size of the insulating layer (5) to the average particle size of the ceramic base material is 1 / 3 or less.

11. The actuator according to claim 1 or 2, characterized in that: The average particle size of the insulating layer (5) is 5 μm or smaller.

12. The actuator according to claim 1 or 2, characterized in that: The average particle size of the insulating layer (5) is 1 μm or smaller.

13. The actuator according to claim 1 or 2, characterized in that: The average particle size of the ceramic base material decreases continuously beyond the insulating layer (5).

14. The actuator according to claim 1 or 2, characterized in that: The insulating layer (5) is made of piezoelectric material.

15. The actuator according to claim 1 or 2, characterized in that: The insulating layer (5) is made of piezoelectric ceramic material.

16. The actuator according to claim 1, characterized in that: The insulating layer (5) consists of at least two layers.

17. The actuator according to claim 1, characterized in that: The insulating layer (5) consists of at least three layers.

18. The actuator according to claim 1, 16 or 17, characterized in that: The total thickness of the insulating layer (5) is 500 μm or less.

19. The actuator according to claim 1, 16 or 17, characterized in that: The total thickness of the insulating layer (5) is 100 μm or less.

20. The actuator according to claim 1, 16 or 17, characterized in that: The total thickness of the insulating layer (5) is 60 μm or less.

21. A method for manufacturing an actuator according to any one of the preceding claims, comprising the steps of: A: A stacked assembly (1) is formed along the stacking axis by electrodes (2) and a ceramic base material with electromechanical properties, such that the stacked assembly (1) has two end sides oriented perpendicular to the stacking axis and at least one side surface extending between these end sides. B: The stacked assembly (1) is provided with a ceramic insulating layer (5) having a material composition different from that of the ceramic base material, such that the ceramic insulating layer (5) extends from one end face to the other on at least one side surface, wherein the material of the ceramic insulating layer (5) is doped with a grain growth inhibitor, such that the structure of the insulating layer (5) has a smaller average grain size than that of the ceramic base material, and in the region where the ceramic insulating layer (5) and the ceramic base material meet, the average grain size of the ceramic base material decreases toward the ceramic insulating layer (5).

22. The method of claim 21, comprising at least one of the following sub-steps: A1: A stacked assembly (1) is composed of an electrode (2) and a film preform made of ceramic base material. A2: The stacked assembly (1) is sintered to transform the ceramic base material into a solid-phase ceramic structure.

23. The method according to claim 21 or 22, comprising at least one of the following sub-steps: B1: Apply at least one, multiple, or all layers of the ceramic insulating layer (5) to the stacked assembly (1) by means of coating, injection molding, plasma spraying, impregnation encapsulation, spraying, and / or using the sol-gel method. B2: The ceramic insulating layer (5) and the ceramic base material are sintered to transform the ceramic insulating layer (5) and the ceramic base material into a solid-phase ceramic structure. B3: Adjust the average particle size of the insulation layer (5) and the average particle size of the ceramic base material by selecting different materials for the ceramic base material and the ceramic insulation layer (5) and / or by selecting process parameters during sintering.

24. The method according to claim 21 or 22, comprising the following steps: C: Polarize the ceramic base material in order to adjust the electromechanical characteristics of the actuator.

25. The method according to claim 21 or 22, comprising the steps of: C: Polarize the ceramic insulating layer (5) in order to adjust the electromechanical characteristics of the actuator.

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