Piezoelectric actuators and microfluidic devices

Abstract

The present invention provides piezoelectric actuators and microfluidic devices, such as microvalves or micropumps. [Solution] The piezoelectric actuator (100) comprises a flexible actuator membrane (102) and at least one piezoelectric element (104A, 104B) which is operable to perform expansion and contraction motion (106) in response to an electric field applied to the piezoelectric element, wherein the piezoelectric element is attached to a portion of the actuator membrane to exert a mechanical force on the membrane, leaving the central region of the membrane open and having a peripheral shape that does not match the outer shape of the actuator membrane.

Description

Technical Field

[0001] The present invention relates to piezoelectric actuators and microfluidic devices, such as microvalves or micropumps.

Background Art

[0002] For decades, piezofilm actuators have been used to provide precise movement, high force, and energy-saving performance. To improve the performance of these actuators and extend their lifespan, several configurations such as unimorphs, bimorphs, stack actuators, etc. have been proposed. Most of these actuators are in the form of a multilayer structure of a piezo material as an active part mounted on other materials such as metal, polymer, or glass as a passive film. Due to its essential electromechanical characteristics, there is strain generated in the piezo material when an external voltage is applied, and as a result, the strain in the piezo material is restricted by the stationary layer, leading to a bending moment. To improve the performance of these actuators, many studies have been conducted, for example, by optimizing the ratio of the bending moment to the rigidity of the multilayer actuator. Others have tried to optimize the applied electric field by changing the metallization layer and electrodes of the actuator in various arrangements or spirals.

[0003] Figures 29 to 31 show a conventional piezoelectric actuator 10 in three different operating states. To develop a buckling actuator membrane 12, it is known to sandwich the membrane 12 (often also referred to as a "diaphragm") between two piezoelectric rings 14A, 14B, and the membrane 12 and the piezoelectric elements 14A, 14B have a flat structure with a certain thickness, and the membrane 12 has the same outer dimensions as the outer dimensions of the piezo elements 14A, 14B (see also, for example, publications [1] to [9]).

[0004] Furthermore, the publication

[10] discloses a piezoelectric actuator comprising a piezoelectric element that expands / contracts according to an electric field state, a sheet portion on which the piezoelectric element is attached to at least one surface, and a support member that supports the piezoelectric element and the sheet portion, wherein the piezoelectric element and the sheet portion vibrate up and down according to the expansion / contraction motion of the piezoelectric element. The sheet portion is connected to the support member via a vibrating film that is not as rigid as the sheet portion. However, the surrounding member is the only passive material that provides the surrounding support and is not an active member that causes or controls bending moments and buckling forces.

[0005] Furthermore, the publication

[11] aims to provide a method for compensating for the low force generation and small displacements that are drawbacks of conventional piezoelectric bending actuators. Furthermore, this paper aims to provide a piezoelectric bending actuator in which the force generation and displacement can be precisely controlled. According to one embodiment, the piezoelectric bending actuator includes a piezoelectric plate containing a piezoelectric ceramic in the shape of a bar, a base plate mounted below the piezoelectric ceramic and longer than the piezoelectric ceramic, and compression members located at both ends of the piezoelectric plate and parallel to the longitudinal direction, capable of compressing the piezoelectric plate. The piezoelectric plate buckles in one direction when the compression members compress both sides of the piezoelectric plate in the absence of an electric field. The piezoelectric plate buckles in the other direction in the presence of an electric field. However, the compression members are not piezoelectric elements.

[0006] A publication

[12] relates to piezoelectric energy harvesting using a nonlinear buckling beam. The energy harvester includes a frame having a base, a first side member attached to the base, and a second side member attached to the base and spaced apart from the first side member. A beam is coupled between the first side member and the second side member of the frame. The beam has a substrate layer having a first end attached to the first side member of the frame, a second end attached to the second side member of the frame, a first face, and a second face opposite the first face. The substrate is elastically deformable in response to vibrational forces. The beam further includes a first piezoelectric layer bonded to the first face of the substrate layer and having terminals for electrical connection to a load, the first piezoelectric layer comprising at least one piezoelectric patch. The frame described in Figure 1 of reference

[12] merely provides a passive, rigid support that creates buckling and does not have an active role in adding or controlling bending and buckling.

[0007] Finally, publication

[13] discloses a piezoelectric micropump in which the pump chamber is isolated by a diaphragm. A piezoelectric element is disposed on the back of the diaphragm, and the diaphragm is deformed by the bending deformation of the piezoelectric element, thereby changing the volume of the pump chamber and transporting the fluid within the pump chamber. A support member is provided to support the back of the piezoelectric element, such that the support member prevents bending of the periphery of the diaphragm in the opposite direction. In this way, the support member prevents the piezoelectric element from floating. Thus, the displacement of the piezoelectric element can be reliably transmitted as a change in the volume of the pump chamber, thereby enhancing the fluid transport performance. However, member 1d shown in Figure 8 of reference

[13] describes a block (support member) and does not describe the active piezoelectric element.

[0008] Several other essential aspects of piezoelectric ceramics seem to limit their application when large deflections are required. One is the natural stiffness of these materials, often in ceramic form, where aluminum has stiffness levels of tens of GPa within the same range. Thus, the piezoelectricity itself contributes significantly to the undesirable bending stiffness of the actuator, thereby reducing the overall deflection achievable in the structure. In addition, the high stiffness corresponds to large stresses generated within the material even with small strains, which is where the second essential characteristic of these materials—the brittle nature of these materials—comes in, making it very difficult to achieve large deflections. In other words, even if a structure can achieve large deflections despite the high bending stiffness of piezoelectric materials, the brittle nature of these materials prevents strains exceeding tens of percent from being reached. [Overview of the project] [Problems that the invention aims to solve]

[0009] Therefore, there is a further need for improved piezo-driven concepts that combine buckling force and bending moment to achieve greater deflection and increase the lifespan of the piezoelectric actuator. [Means for solving the problem]

[0010] This objective is addressed by the subject matter of the independent claim. Advantageous embodiments of the present invention are the subject matter of the dependent claim.

[0011] This invention is based on the finding that when the membrane and the piezoelectric element do not have the same external shape, the actuator can be developed with better control by combining bending and buckling forces to achieve higher performance in terms of deflection and force / pressure capacity. Instead, the dimensions of the membrane are smaller or larger than the dimensions of the piezoelectric element (hereinafter also referred to as the "piezo element" in some cases).

[0012] Advantageously, according to the present invention, the actuator is designed to combine buckling and bending forces in both deflection directions of the actuator membrane during operation. In other words, compressive forces can be generated in both deflection directions in addition to bending forces.

[0013] More specifically, the piezoelectric actuator according to the present invention comprises a flexible actuator membrane and at least one piezoelectric element, which is operable to perform expansion and contraction movements in response to an electric field applied to the piezoelectric element, wherein the piezoelectric element is attached to a portion of the actuator membrane to exert a mechanical force on the membrane, and the piezoelectric element has a peripheral shape that does not coincide with the outer shape of the actuator membrane, leaving the central region of the membrane open.

[0014] First, the film can have a diameter smaller than the outer diameter of the piezoelectric element. More specifically, the piezoelectric element may include at least one ring-shaped flat plate attached to the film such that the ring-shaped peripheral region of the film is covered by the ring-shaped inner region of the piezoelectric element.

[0015] The advantages of smaller membrane dimensions can be more easily explained by energy analysis techniques. The energy of the piezoelectric material results in the deformation energy of the actuator and the external work done by the actuator (such as replacing liquid). Thus, when the membrane has a larger area (as referenced in [1] to

[10] ), the deformation energy is consumed in compressing and deforming the membrane (the entire membrane) and the corresponding adhesive that transmits stress from the piezo ring to the membrane over a larger area. By giving a smaller membrane area, the deformation energy is used more efficiently in the accessible region of the membrane, resulting in more deflection and achieving more external work. Thus, since the most important area of ​​the membrane doing the actual mechanical work is the region inside the piezo section, ideally reducing the membrane size to just this region can significantly improve performance in addition to reducing the cost of the material.

[0016] Alternatively, the dimensions of the membrane may be greater than those of the piezoelectric element. This extension of the membrane beyond the dimensions of the piezoelectric element can be used for mounting and support purposes, creating longer moment arms compared to when the mounting and support area is limited to the piezo dimensions, and thus allowing for greater achievable deflection. In detail, the piezoelectric element comprises at least one ring-shaped flat plate attached to the membrane, having an outer diameter smaller than the outer diameter of the membrane.

[0017] In the context of this invention, “ring-shaped” means a structure that surrounds an open center with a closed or partially interrupted area having an inner and outer outline. More specifically, the ring shape is not limited to a circular outline and includes any of the outlines, for example, elliptical, rectangular, or irregular. Furthermore, it should be noted that neither the film nor the piezoelectric element needs to be symmetrical in the plane of the film. The inner and outer outlines of the piezoelectric element do not need to have central similarity to each other.

[0018] Furthermore, the piezoelectric element may comprise first and second ring-shaped plates, which differ from each other in terms of their outer and / or inner diameters, and which are attached to opposing circumferential surfaces of the film. Such a configuration generates an asymmetric force.

[0019] Several techniques can be considered to generate asymmetric forces in two piezoelectric elements. Alongside the more obvious method of applying asymmetric voltages to the two piezoelectric elements, the dimensions of the piezoelectric elements can differ from each other, thereby further emphasizing the generation of asymmetry in the desired direction. By causing greater radial contraction in one piezoelectric element compared to the other, a larger force multiplied by the distance to the neutral axis is generated by the larger piezo ring, creating a net bending moment in addition to the buckling force.

[0020] Since the force generated in a piezoelectric element can be changed by changing its thickness, the thickness of the two piezoelectric rings can be changed to create an asymmetric force between them; in other words, the first and second ring-shaped flat plates can have different thicknesses. This is due to two effects: firstly, the change in the generated electric field in response to the applied voltage, and secondly, the stress capacity of the piezoelectric element, which varies with thickness. When the same electric field is applied to two piezoelectric sections, a greater force is generated in the piezoelectric element with the greater thickness, and therefore the resulting bending moment is created along with the net buckling force.

[0021] Such asymmetry can also be achieved by placing the piezo ring on only one side of the membrane. Compared to the configuration described above, this configuration results in a more pronounced bending moment because there is no opposition from opposing piezos. However, the first drawback of such a configuration is the reduction in buckling force. Another drawback is the reduction in bending control, because, considering that these forces are applied at a distance from the neutral axis of the structure, the bending moment in this configuration is limited to that generated by the buckling force (or the tension of the piezo if the piezo is activated to move in the opposite direction to the inward buckling direction). It should be noted that this particular cross-section is not limited to circular actuators. The same concept may be applied to other shapes such as rectangular and elliptical, or other non-homogeneous or arbitrary forms that may have the same buckling effect.

[0022] Advantageously, the membrane may comprise multiple sections, each made of a different material or even an empty region. For example, the membrane may have a central section made of a material with high rigidity (Young's modulus) (to have higher force and pressure capacity) and an outer section made of another, more flexible material. This allows for the application of more flexible boundary conditions and thus the realization of non-zero gradients at the mounting and support positions, thereby resulting in greater final deflection. Furthermore, the gap between the first ring-shaped plate and the second ring-shaped plate is preferably at least partially filled with a flexible spacer material.

[0023] The spacer material may also have the shape of a ring connected to the membrane by one or more radially extending webs. Such an arrangement has the advantage that the spacers can be more easily aligned and assembled. The spacers and membrane may be cut from the same material to reduce manufacturing and assembly steps.

[0024] According to a further advantageous embodiment of the present invention, the piezoelectric element comprises at least one ring-shaped flat plate attached to the film in contact with the outer circumferential end face of the film. In other words, two essentially flat buckling piezoelectric elements may be replaced by a single piece (for example, in the case of a circular cross-section, two piezo rings may be replaced by one piezo tube), thereby applying pure tension or compression (buckling) to the structure and thus amplifying the deflection and force produced by the bending piezo which may be located in the central portion of the film.

[0025] When the piezoelectric element is attached to only a portion of the outer peripheral end face of the film, this buckling piezoelectric (such as a tube) is positioned asymmetrically with respect to the neutral axis of the film, thus resulting in a combination of bending moment and buckling in the film.

[0026] Furthermore, the membrane can include regions with material modification such that the piezoelectric element does not exert a force on the membrane and the membrane has a predetermined deflected shape. According to an advantageous embodiment of the invention, pre-buckling in the membrane can be brought about by applying local changes to the material of the membrane, such as the laser treating the surface, which can result in various ranges of physical or chemical changes, for example, thermal expansion, oxidation, material / alloy phase changes, etc. This results in various effects such as local volume changes, expansion, or concentration of pre-stress in the material, and this can result in a pre-stress / pre-strain state, which may cause pre-buckling in the membrane. The amount of pre-buckling can be changed / regulated by various laser processing parameters (e.g., pulse rate, frequency, output, etc.), geometric parameters (location to be targeted, pattern, number of repetitions, etc.), thermal parameters, and other parameters.

[0027] Furthermore, the membrane may have a thickness that varies along the radial direction of the membrane and / or may include regions made of different materials.

[0028] To provide mechanical support, the piezoelectric element may further include a support element that mechanically fixes the membrane.

[0029] According to an advantageous embodiment, the piezoelectric actuator may further include at least one bending piezoelectric element that is attached to the membrane within the central region of the membrane and applies a bending moment to the membrane. Such a bending piezo can be added to the central portion of the membrane to combine the "conventional bending configuration" with the buckling technique. The additional bending moment applied by the "bending piezo" (whether by expansion or contraction) increases the forces and pressures that the actuator can handle.

[0030] Bending piezos can also eliminate the need for a double buckling piezo configuration by providing the bending moment necessary to control the actuator (by contraction or expansion) in the desired direction. And the buckling force from the sides has precisely the amplification effect on force and deflection.

[0031] For example, several bending piezoelectric elements may be present, one or more of which buckle, at different positions on the membrane to create more complex shapes, or on both sides of the membrane to generate more bending moments. In this case, the bending moment can be increased, which simplifies, for example, the flow control of a proportional valve, as it lowers the buckling branch point, especially when the membrane is near its middle (initial) position.

[0032] It may be advantageous to provide flexible support means for mounting the actuator membrane. Such flexible mounting, as an alternative to fixed clamping of the membrane, allows for a non-zero gradient within the peripheral region of the membrane. This results in greater final deflection of the membrane. For example, a pair of elastic O-rings may be used to support the membrane. However, other suitable flexible mounting means can also be used.

[0033] The present invention also relates to a microfluidic device comprising a piezoelectric actuator according to the present invention. For example, the microfluidic device may be a microvalve having at least two fluid ports and a valve seat portion positioned around one of the ports and protruding from the port toward a membrane, thereby enabling a flexible membrane to open and close each fluid path by contacting and moving away from the valve seat portion. The present invention further provides a microfluidic device that can operate as a micropump, wherein a moving actuator pushes fluid out of at least one of the ports.

[0034] Furthermore, at least one compressible, preferably elastic, sealing layer may be provided between the membrane and the valve seat portion. This layer serves the purpose of sealing and can be attached to the surface of the valve seat portion, the surface of the membrane, or both surfaces. For example, a laminated silicone layer may be applied as the sealing layer.

[0035] However, all mentions of micropump and microvalve applications merely highlight the advantages in tuning performance and do not limit applications to these areas. Other applications may include adaptive optics (e.g., adaptive lenses and adaptive mirrors), precision motion, and MEMS acoustics (e.g., microphones and speakers).

[0036] When used herein, "rigid" means rigid and inflexible; that is, a rigid structure is not made to be deformable during normal use of the structure.

[0037] As used herein, "flexibility" means not rigid or inflexible; that is, the material can withstand elastic deformation and return to its original shape as long as it does not enter a plastic region.

[0038] When used herein, "stretchable" means resilient, i.e., elastically deformable with elongation. A stretchable structure is made to be elastically deformable (with elongation) during normal use.

[0039] When used herein, "compressible" means elastic, i.e., it means that it is elastically deformable in the direction of applied pressure, with a decrease in size. Compressible structures are made to be elastically deformable (with a decrease in dimensions) during normal use.

[0040] The accompanying drawings are incorporated herein and form part of this specification to illustrate embodiments of the present invention. These drawings, together with the specification, serve to illustrate the principles of the present invention. The drawings are merely for illustrating preferred and alternative examples of how the present invention may be constructed and used, and should not be construed as limiting the present invention to the illustrated and described embodiments only. Furthermore, several aspects of the described embodiments can form solutions according to the present invention (individually or in different combinations). Further features and advantages will become apparent from the following more detailed description of various embodiments of the present invention, as shown in the accompanying drawings (where the same reference numerals indicate the same elements). [Brief explanation of the drawing]

[0041] [Figure 1] This is a schematic cross-sectional view of a piezoelectric actuator according to the first embodiment. [Figure 2] This is a schematic cross-sectional view of a piezoelectric actuator according to a second embodiment. [Figure 3] This is a schematic cross-sectional view of a piezoelectric actuator according to a first embodiment having a first film structure with varying thickness. [Figure 4] This is a schematic cross-sectional view of a piezoelectric actuator according to a first embodiment having a second film structure with varying thickness. [Figure 5] This is a schematic cross-sectional view of a piezoelectric actuator according to a further embodiment. [Figure 6] This is a schematic cross-sectional view of a piezoelectric actuator according to a further embodiment. [Figure 7] This is a schematic cross-sectional view of a piezoelectric actuator according to a further embodiment. [Figure 8] This is a schematic top view of a piezoelectric actuator in a further embodiment. [Figure 9] This is a schematic cross-sectional view of a piezoelectric actuator according to a further embodiment. [Figure 10] This is a schematic cross-sectional view of a piezoelectric actuator according to a further embodiment. [Figure 11] This is a schematic cross-sectional view of a piezoelectric actuator according to a further embodiment. [Figure 12] This is a schematic cross-sectional view of a piezoelectric actuator according to a further embodiment. [Figure 13] This is a schematic cross-sectional view of a piezoelectric actuator according to a further embodiment. [Figure 14] This is a schematic cross-sectional view of a piezoelectric actuator according to a further embodiment. [Figure 15] This is a schematic cross-sectional view of a piezoelectric actuator in a further configuration in a deflected position. [Figure 16] This is a schematic cross-sectional view of the piezoelectric actuator in the neutral position shown in Figure 15. [Figure 17] This is a schematic cross-sectional view of a piezoelectric actuator according to a further embodiment. [Figure 18] This is a schematic cross-sectional view of the film according to a further embodiment. [Figure 19] This is a schematic cross-sectional view of the film according to a further embodiment. [Figure 20] This is a schematic cross-sectional view of the film according to a further embodiment. [Figure 21] This is a schematic cross-sectional view of the film according to a further embodiment. [Figure 22] This is a schematic cross-sectional view of a piezoelectric actuator according to a further embodiment. [Figure 23] This is a schematic cross-sectional view of a piezoelectric actuator according to a further embodiment. [Figure 24] This is a schematic cross-sectional view of a piezoelectric actuator according to a further embodiment. [Figure 25] This is a schematic cross-sectional view of a microvalve in an advantageous configuration in a first operating state. [Figure 26] This is a schematic cross-sectional view of the microvalve in the second operating state shown in Figure 25. [Figure 27] Figure 25 is a schematic cross-sectional view of the micro valve in a further operating state. [Figure 28] Figure 25 is a schematic cross-sectional view of the micro valve in a further operating state. [Figure 29] This is a schematic cross-sectional view of a known piezoelectric actuator in different operating states. [Figure 30]This is a schematic cross-sectional view of a known piezoelectric actuator in different operating states. [Figure 31] This is a schematic cross-sectional view of a known piezoelectric actuator in different operating states. [Modes for carrying out the invention]

[0042] The present invention will now be described in more detail with reference to the drawings, particularly Figure 1.

[0043] Figure 1 shows a schematic cross-sectional view of a piezoelectric actuator 100 according to a first exemplary embodiment of the present invention. The piezoelectric actuator 100 comprises a flexible membrane 102 and two ring-shaped piezoelectric elements 104A and 104B. As indicated by arrow 106, piezoelectric element 104 exerts an inwardly directed force when an electric field is applied to the piezoelectric element. Throughout this disclosure, the electrodes of the piezoelectric elements are formed according to their polarization, as is known to those skilled in the art, so as to cause the piezoelectric elements to exert inward or outward forces (compression and tension), but it should be noted that these are not shown in the drawings to avoid ambiguity of the concept.

[0044] Furthermore, the piezoelectric element can be mechanically connected to the film in any suitable way, for example, by an adhesive layer.

[0045] According to the present invention, the film 102 has dimensions smaller than the piezoelectric element 104. In other words, the piezoelectric element is in mechanical contact with the film in an area much smaller than the surface of the piezoelectric element. For example, the piezoelectric elements 104A and 104B can be attached to the film 102 occupying approximately 20% of the total area. However, other proportions are of course possible, and it should be noted that not all figures are to the same scale, both in terms of thickness and distance.

[0046] For example, by combining bending and buckling of the membrane 102 in the same manner as shown in Figures 29 to 31, better control is possible in terms of deflection and force and / or pressure capabilities, and higher performance can be achieved.

[0047] The advantages of smaller membrane dimensions can be more easily explained by energy analysis techniques. The energy of the piezoelectric material results in the deformation energy of the actuator and the external work done by the actuator (such as transferring liquid). Therefore, when the membrane has a larger area, the deformation energy is consumed in compressing and deforming the larger area of ​​membrane 102, i.e., the entire membrane, as well as the corresponding adhesive that transmits stress from the piezo ring to the membrane. In contrast, for smaller membrane areas (as shown in Figure 1), the deformation energy is used more efficiently within the central region of membrane 102, resulting in greater deflection and the realization of more external work. Therefore, since the most important area of ​​the membrane doing the actual mechanical work is the region inside the piezo section, ideally reducing the membrane size to just this region can significantly improve performance in addition to reducing the cost of the material.

[0048] Furthermore, the membrane 102 may have an initial curvature due to pre-stressing, such as pre-bending or pre-buckling, so as to generate asymmetric deflection / force in a desired direction. In addition, this curvature reduces the force required to achieve a certain degree of deflection.

[0049] On the other hand, as shown in Figure 2, the piezoelectric actuator 200 may include a membrane 202 with a larger diameter than the piezoelectric element 204. This extension of the membrane beyond the dimensions of the piezoelectric element can be used for mounting / support (as shown in Figure 2), creating longer moment arms (compared to when the mounting / support area is limited to the piezo dimensions), and thus enabling greater deflection. In Figure 2, the support element 208 is schematically shown as an O-ring. Of course, it will also be apparent to those skilled in the art that any suitable form of the support element 208 is available.

[0050] Further advantageous modifications of the piezoelectric actuator 300 are shown in Figure 3, where the film 302 has a reduced thickness in the center compared to the peripheral region. On the other hand, Figure 4 shows a piezoelectric actuator 400 with a film 402 that is thicker in the center compared to the edge region in contact with the piezoelectric element 404.

[0051] Since the thickness of the film provides a third power to the bending stiffness, the increased thickness in the center (as in the example shown in Figure 4) results in a higher moment transmitted through the film, and therefore a higher operating pressure, i.e., the actuator 400 can deflect under higher pressure and / or force. The disadvantage is that the deflection is reduced. This increase in thickness can also be carried out, for example, by adding / attaching other layers of the same or different material to the film 402.

[0052] While increasing thickness can lead to a decrease in deflection, it should be noted that, depending on the design parameters, this can result in a greater volumetric displacement.

[0053] The reduction in the thickness of the membrane 302 in the center results in greater deflection, at the expense of reducing the operating pressure and generated force due to the decrease in the bending stiffness of the membrane.

[0054] In all applications, the ability to adjust for greater deflection or greater pressure / force can provide designers with the option to optimize desired characteristics. For example, in micropump applications, greater actuator deflection can result in higher flow rates. In microvalve applications, greater deflection in the actuator allows the valve to achieve a higher flow coefficient.

[0055] On the other hand, the increased pressure and force capabilities of the actuator can lead to higher back pressure in micropump applications, or a wider operating pressure range in microvalve applications.

[0056] Several techniques may be used to generate asymmetric forces in two piezoelectric elements to further emphasize the resulting deflection / force in a particular direction. Alongside the more obvious technique of applying asymmetric voltages to the two piezoelectric elements, the dimensions of the piezoelectric elements may differ from each other to further emphasize the generation of asymmetry in the desired direction. By causing one of the piezoelectric elements to contract more radially than the other (as indicated by the black arrow), a larger force multiplied by the distance to the neutral axis is generated by the larger ring-shaped piezoelectric element 504B, thereby generating a net bending moment (indicated by arrow 510 in Figure 5) alongside the buckling force.

[0057] A further example of a piezoelectric actuator 600 in which an asymmetric force is generated by a piezoelectric element 604 is shown in Figure 6.

[0058] In this example, the two opposing piezoelectric elements 604A and 604B have different thicknesses, while their diameters are equal. It should be noted that even more different diameters are possible. Using piezoelectric elements 604A and 604B with different thicknesses results in asymmetric forces due to two factors. Firstly, the electric field generated by a given applied voltage changes. Secondly, the stress characteristics of the piezoelectric elements 604A and 604B change according to their thickness. Figure 6 schematically illustrates this effect. When the same electric field is applied to piezoelectric elements 604A and 604B, a greater force 606B is generated in the thicker piezoelectric element 604B. Therefore, the resulting bending moment 610 is generated along with the net buckling force.

[0059] A further advantageous example of the piezoelectric actuator 700 according to the present invention is shown in Figure 7.

[0060] As shown in this figure, the asymmetry of the driving force can be achieved by providing a ring-shaped piezoelectric element 704 on only one side of the film. Compared to the configuration described earlier, this configuration results in the generation of a larger bending moment 710 by lacking opposition from another piezoelectric element. However, the configuration shown in Figure 7 has the disadvantage of generating a low buckling force. Another disadvantage is reduced bending control, which is because, in this configuration, considering that these forces are applied at a distance from the neutral axis of the structure, the bending moment 710 is limited to that generated by the buckling force (or the tension of the piezoelectric element 704 if the piezoelectric element 704 is activated to move in the opposite direction to the inward buckling direction).

[0061] As schematically shown in the top view of Figure 8, of course, the piezoelectric actuator 800 according to the principle of the present invention is not limited to a circular shape. As shown in Figure 8, the same concept may be applied to other shapes such as rectangular and elliptical shapes, or other non-homogeneous or arbitrary forms that may have the same buckling effect.

[0062] According to a further advantageous example of this disclosure, the membrane may comprise multiple sections, each comprising different materials or even voids. As shown in Figure 9, for example, a material with high rigidity, i.e., a high Young's modulus, may be provided in the central region 912 of the membrane 902 to achieve higher force and pressure capacity. Another more flexible material may be used in the peripheral region 914. This allows for the application of more flexible boundary conditions around the periphery of the membrane 902. Thus, it is possible to achieve a non-zero gradient at the location where the mounting means (support means) 908 is provided. This results in a higher final deflection of the membrane.

[0063] Alternatively, as shown in Figure 10, a preferably flexible spacer 1016 may be provided between the piezoelectric elements 1004A and 1004B to provide the application of more flexible boundary conditions around the film 1002. In this example, the support means 1008 is provided within the region of the piezoelectric elements 1004A and 1004B. The area between the piezoelectric elements 1004A and 1004B is filled with the spacer 1016 to act as a more flexible support without hindering the transmission of inwardly directed forces of the piezoelectric elements 1004A and 1004B that must be transmitted to the central portion of the film 1002.

[0064] A further advantageous example of the piezoelectric actuator 1100 is shown in Figure 11. In this example, the material properties of the film 1102 vary, for example, radially. The material properties may be modified by further processing, for example, by heat treatment such as annealing and sintering, laser ablation, alloy phase change, etc. Figure 11 shows an example in which the central portion 1112 of the film 1102 is hardened. In the case of applications in microvalves, this hardening of the central region 1112 increases the leak-proof area established between the central portion of the film 1102 and the valve seat, thereby increasing the dynamic operating pressure of the valve.

[0065] As shown in Figure 11, the piezoelectric elements 1104A and 1104B exert different radial forces 1106A and 1106B on the film 1102. Therefore, in addition to more buckling force, a bending force 1110 is generated.

[0066] Figures 12 to 16 show a further advantageous embodiment of the present invention in which at least one bending piezoelectric element is provided in addition to the piezoelectric element that generates the buckling force. By adding such a bending piezoelectric element to the central region of the film, the conventional bending configuration can be combined with more modern buckling techniques. The additional bending moment is applied by expansion or contraction by the bending piezoelectric element.

[0067] As shown in Figure 12, one such bending piezoelectric element 1214 is attached to the central region 1212 of the film 1202. Thus, the film 1202 of the piezoelectric actuator 1200 is driven radially by the two piezoelectric elements 1204A and 1204B and further bent by the contraction and expansion (represented by the double arrow 1216) of the bending piezoelectric element 1214. Thus, the force and pressure responsible for the capabilities of the piezoelectric actuator 1200 are improved.

[0068] As shown in Figure 13, by attaching the bending piezoelectric element 1314 to the central region 1312 of the film 1302, it is even possible to eliminate the need for two opposing piezoelectric elements positioned on both sides of the film. Instead, a single piezoelectric element 1304 is positioned on one side of the film 1303. In this case, the bending moment 1310 necessary to control the actuator 1300 in the desired direction is created by the expansion and contraction of the bending piezoelectric element 1314. In Figure 13, for example, the expansion of the bending piezoelectric element 1314 causes the film 1302 to bend upward. The buckling force acting from the side toward the center amplifies the effect on the force and the resulting deflection.

[0069] According to a further aspect of the present disclosure shown in Figure 14, a pair of opposing buckling piezoelectric elements is replaced by a single piezoelectric element 1404. For example, a tubular piezoelectric element 1406 may be provided to induce a radially directed buckling force 1406 in the film 1402. This tubular piezoelectric element 1404 interacts with the circumferential surface 1418 of the film 1402 instead of the upper or lower surface 1420, 1422 (as shown for the previous example).

[0070] Therefore, the piezoelectric element 1404 applies a pure tensile or compressive (buckling) force to the film 1402 such that the force and deflection produced by the bending piezoelectric element 1414 located in the central region 1412 of the film 1402 are amplified.

[0071] As further shown in the piezoelectric actuator 1500 shown in Figures 15 and 16, there may be several bending piezoelectric elements 1514 in combination with buckling piezoelectric elements 1504 (or 1204), for example, at different positions on the film 1502 to create more complex shapes, or on both sides of the film 1502 (as shown in Figures 15 and 16) to generate more bending moments. In this case, the bending moment can be increased, which simplifies, for example, the flow control of a proportional valve, as it lowers the buckling branch point, especially when the film is near its middle (initial) position. Figures 15 and 16 show an example of this configuration in two operating states (deformed state and neutral state, respectively). The bending piezoelectric elements 1514A and 1514B are located centrally and symmetrically on the film 1502 in this example. The same bending direction is applied by one of the bending piezoelectric elements 1514A expanding and the other 1514B contracting.

[0072] Figure 17 shows a further advantageous example of the piezoelectric actuator 1700. In this example, a single buckling piezoelectric element 1704 (such as a tube) is positioned asymmetrically with respect to the neutral axis of the film 1702 in order to create a combination of buckling force 1706 and bending moment 1710 in the film 1702.

[0073] As explained with reference to Figures 18 to 21, pre-buckling in film 1802 can be achieved by inducing localized changes in material properties.

[0074] For example, laser treatment of the film surface is performed to induce a range of physical or chemical changes, such as thermal expansion, oxidation, material and / or alloy phase changes. This results in different effects in the material, such as localized volume changes, expansion, or pre-stress concentrations. These effects can be used to create pre-stress or pre-strain states, which also induce pre-deflection, such as pre-buckling, in the film 1802. For example, Figure 18 shows an initially flat film 1802 with the initial state of a region 1824 that is to be affected by a special treatment such as laser processing.

[0075] These areas 1824 are then expanded as shown in Figure 19, and their distance from the neutral axis 1826 generates a radial force that also brings about a bending moment 1810 around this axis. The result is a film 1802 that is deformed into a form of pre-buckling and / or pre-bending. The amount of pre-buckling can be changed and adjusted by various laser processing parameters (e.g., pulse rate, frequency, power, etc.), geometric parameters (target location, pattern, number of repetitions, etc.), thermal parameters, and other parameters.

[0076] Furthermore, as shown in Figure 20, the buckling force 1806 can also be increased, but the bending moment 1810 can be decreased, by exposing the other side of the film. Thus, different magnitudes and states of pre-buckling can be realized. The film 1802 can be treated to be monostable (having only one stable equilibrium on one side) or bistable (having two stable equilibrium states on both sides). More complex shapes or even other modes of pre-buckling can be realized using more complex combinations, such as different laser patterns, as schematically shown in Figure 21.

[0077] As shown for the piezoelectric actuators 2200 and 2300 in Figures 22 and 23, the films 2202 and 2302 may be formed to have an initial curvature due to pre-stressing such as pre-bending, pre-buckling, or material treatment, as shown in Figures 18 to 21. Such an initial curvature results in asymmetric deflection and force in the desired direction. Furthermore, this curvature reduces the force required to achieve a certain degree of deflection during operation.

[0078] Figure 24 shows a further advantageous embodiment of the piezoelectric actuator 2400 according to the present invention. In this example, the membrane 2402 includes a central portion 2412 which is hardened to enhance the force that can be exerted on the valve seat portion, for example.

[0079] As an exemplary application of a piezoelectric actuator, Figures 25 to 28 show a fluid control device 2500 comprising a piezoelectric actuator 1000 as shown in Figure 10. In this example, the microfluidic control device 2500 has a base 2528 and a cover 2530. The piezoelectric actuator 1000 is fixed between the base 2528 and the cover 2530 by two elastic O-rings 1008 held in a notch 2532. The base 2528 includes an inlet 2534 and an outlet 2536. A valve seat portion 2538 surrounds the inlet 2534, and a membrane 1002 is positioned near the valve seat portion 2538, thereby using the downward displacement of the membrane 1002 to interact with the valve seat portion and close the fluid path from the inlet 2534.

[0080] Figure 25 shows the fluid control device 2500 in its initial state. By contracting one piezoelectric element and expanding the other, the actuator 1000 bends toward the expanded piezoelectric element, as shown in Figure 26, and the actuator bends toward the valve seat portion 2538. Advantageously, as can be seen from this figure, the flexible support provided by the O-ring 1008 allows for a non-zero gradient of the actuator and therefore the membrane. This allows for a higher final deflection of the membrane, instead of the fixed boundary conditions of a tightly fastened cantilever. The O-ring acts as both a hinge and a sealant. Of course, other flexible support means, such as a laminated silicone layer, can also be used instead of the pair of O-rings shown in Figures 25 to 28.

[0081] The actuator 1000 can buckle in the direction it is already bent when both piezoelectric elements contract, as shown in Figure 27. Similarly, the actuator 1000 can bend and buckle away from the valve seat portion 2538 to open the fluid connection portion 2540 between the inlet port 2534 and the outlet port 2536, as shown in Figure 28. Since the gap between the membrane 1002 and the valve seat portion 2538 is a critical factor determining the flow through the valve, a proportional valve can be realized in which the flow rate and / or pressure of the fluid can be controlled by controlling the deflection of the actuator and thus the resulting gap.

[0082] Although not shown in the figure, at least one compressible, preferably elastic, sealing layer may be provided between the membrane and the valve seat portion. This layer serves the purpose of sealing and can be attached to the surface of the valve seat portion, the surface of the membrane, or both surfaces. For example, a laminated silicone layer may be applied as the sealing layer.

[0083] Furthermore, the valve seat portion may be formed as a single integrated part having a bottom plate, and therefore, it is a rigid valve seat portion.

[0084] In summary, it should be noted that the piezoelectric element according to this disclosure does not necessarily have a uniform thickness and may have a non-flat shape.

[0085] No limitations are intended regarding adhesion between piezoelectric elements and various films.

[0086] No limitations are intended regarding the polarization direction, the electrode structure of the piezoelectric element, or the direction of the applied voltage. The only important point is that axial compressive buckling force is generated regardless of polarization and applied voltage.

[0087] Furthermore, each piezoelectric element may comprise an array of piezoelectric elements or be manufactured in a stacked configuration in order to decrease the applied voltage while maintaining the same electric field, or to increase the applied force.

[0088] All of the above mentions of micropump and microvalve applications merely highlight the advantages in terms of performance tuning and do not limit applications to these fields. Other applications may include adaptive optics, precision motion, and MEMS acoustics (e.g., MEMS speakers).

[0089] References [1]MC Wapler, M. Sturmer, and U. Wallrabe, “A Compact, Large-Aperture Tunable Lens with Adaptive Spherical Correction,” in 2014 International Symposium on Optomechatronic Technologies, Seattle, WA, 2014, pp. 130-133. [2]M. C. Wapler, C. Weirich, M. Sturmer, and U. Wallrabe, “Ultra-compact, large-aperture solid state adaptive lens with aspherical correction,” in 2015 Transducers - 2015 18th International Conference on Solid-State Sensors, Actuators and Microsystems (Transducers): 21-25 June 2015, Anchorage, Alaska, Anchorage, AK, USA, 2015, pp. 399-402. [3]A. Shabanian, F. Goldschmidtboeing, H. G. B. Gowda, C. C. Dhananjaya, and P. Woias, Eds., The deformable valve pump (DVP): IEEE, 2017. [4]A. Shabanian et al., “A novel piezo actuated high stroke membrane for micropumps,” Microelectronic Engineering, vol. 158, pp. 26-29, 2016. [5]K. Philipp et al., “Spherical aberration correction of adaptive lenses,” in San Francisco, California, United States, 2017, p. 1007303. [6]K. Philipp et al., “Axial scanning and spherical aberration correction in confocal microscopy employing an adaptive lens,” in Optics, Photonics, and Digital Technologies for Imaging Applications V, Strasbourg, France, 2018, p. 12. [7]F. Lemke et al., “Multiphysics simulation of the aspherical deformation of piezo - glass membrane lenses including hysteresis, fabrication and nonlinear effects,” Smart Mater. Struct., vol. 28, no. 5, p. 55024, 2019. [8]F. Lemke, Y. Frey, U. Wallrabe, M.C. Wapler, Ed., Pre - stressed Piezo Bending - buckling Actuators for Adaptive Lenses: 16th International Conference on New Actuators : 25 - 27 June 2018. Frankfurt am Main: VDE, 2018. [9] Shabanian, F. Goldschmidtboeing, C. Dhananjaya, H. Bettaswamy Gowda, E. Baeumker and P. Woias, Ed., Low fluidic resistance valves utilizing buckling actuators. [Frankfurt am Main]: VDE VERLAG GMBH, 2017.

[10] Specification of Chinese Patent No. 101336562

[11] Republic of Korea Patent Publication No. 20140128273

[12] International Publication No. 2018035542 Pamphlet

[13] U.S. Patent No. 8454327 [Explanation of symbols]

[0090] 10 Piezoelectric Actuator 12 Buckling actuator membrane 14A Piezoelectric Ring 14B Piezoelectric Ring 100 Piezoelectric Actuators 102 Flexible membrane, membrane 104 Piezoelectric element 104A Ring-shaped piezoelectric element 104B Ring-shaped piezoelectric element 200 piezoelectric actuators 202 Membrane 204 Piezoelectric element 208 Support elements 300 Piezoelectric Actuator 302 Membrane 400 piezoelectric actuators, actuators 402 Membrane 404 Piezoelectric element 504B Ring-shaped piezoelectric element 510 Bending moment 600 Piezoelectric Actuator 604A Piezoelectric element 604B Piezoelectric element 610 Bending moment 700 piezoelectric actuators, actuators 704 Ring-shaped piezoelectric element, piezoelectric element 710 Bending moment 800 Piezoelectric Actuator 902 Membrane 908 Attachment means (support means) 912 Central area 914 Peripheral area 1000 piezoelectric actuators 1002 Membrane 1004A Piezoelectric element 1004B Piezoelectric element 1008 Support means, elastic O-ring 1016 Flexible spacer, spacer 1100 Piezoelectric Actuator 1102 Membrane 1104A Piezoelectric element 1104B Piezoelectric element 1106A Radial force 1106B Radial force 1110 Bending force 1112 Central part 1200 Piezoelectric Actuator 1202 Membrane 1204A Piezoelectric element 1204B Piezoelectric element 1212 Central area 1214 Bending Piezoelectric Element 1216 Contraction and Expansion 1300 Actuator 1302 Membrane 1303 Membrane 1304 Piezoelectric element 1310 Bending moment 1312 Central area 1314 Bending piezoelectric element 1402 Membrane 1404 Tubular piezoelectric element, piezoelectric element 1406 Tubular piezoelectric element 1406 Buckling force directed radially 1412 Central area 1414 Bending piezoelectric element 1418 Peripheral surface 1420 Top surface 1422 Bottom surface 1500 Piezoelectric Actuator 1502 Membrane 1504 Buckling Piezoelectric Element 1514 Bending Piezoelectric Element 1514A Bending Piezoelectric Element 1514B Bending Piezoelectric Element 1700 Piezoelectric Actuator 1702 Membrane 1704 Buckling Piezoelectric Element 1706 Buckling force 1710 Bending moment 1802 Membrane 1806 Buckling force 1810 Bending moment 1824 regions, areas 1826 Neutral axis 2200 Piezoelectric Actuator 2202 Membrane 2300 Piezoelectric Actuator 2302 Membrane 2400 Piezoelectric Actuator 2402 Membrane 2412 Central part 2500 Fluid control devices, microfluidic control devices 2528 base 2530 cover 2532 cuts 2534 Entrance Port, Entrance 2536 Exit port, exit 2538 Valve seat section 2540 Fluid connection part

Claims

1. Flexible actuator membranes (102, 202), At least one piezoelectric element (104, 204) which is operable to perform expansion and contraction movements in response to an electric field applied to the piezoelectric element (104, 204) and A piezoelectric actuator (100, 200) comprising, The piezoelectric elements (104, 204) are attached to a part of the actuator film (102, 202) so as to exert mechanical force on the film (102, 202). The piezoelectric elements (104, 204) leave the central region of the film (102, 202) open and have peripheral outlines that do not match the outer shape of the actuator film (104, 204), and the piezoelectric actuators (100, 200).

2. The piezoelectric actuator according to claim 1, wherein the piezoelectric elements (104, 204) comprises at least one ring-shaped flat plate attached to the film (102, 202) such that the ring-shaped peripheral region of the film is covered by the ring-shaped inner region of the piezoelectric elements (104, 204).

3. The piezoelectric actuator according to claim 2, wherein the piezoelectric element comprises first and second ring-shaped flat plates (104A, 104B; 204A, 204B), the first and second ring-shaped flat plates differ from each other in terms of their outer diameter and / or inner diameter, and the first and second ring-shaped flat plates are attached to the opposing circumferential surfaces of the film.

4. The piezoelectric actuator according to claim 2 or 3, wherein the first and second ring-shaped flat plates (604A, 604B) have different thicknesses.

5. The piezoelectric actuator according to any one of claims 2 to 4, wherein the gap between the first and second ring-shaped flat plates (1004A, 1004B) is at least partially filled with a spacer material (1016).

6. The piezoelectric actuator according to claim 1, wherein the piezoelectric element (204) comprises at least one ring-shaped flat plate attached to the film (202) and has an outer diameter larger than the outer diameter of the film.

7. The piezoelectric actuator according to claim 1, wherein the piezoelectric element comprises at least one ring-shaped flat plate (1404) that is attached in contact with the film (1402) at its outer peripheral end surface (1418).

8. The piezoelectric actuator according to claim 7, wherein the piezoelectric element (1704) is attached only to a part of the outer peripheral end surface of the film (1702).

9. The piezoelectric actuator according to any one of claims 1 to 8, wherein the film (302, 402) has a thickness that varies along the radial direction of the film.

10. The piezoelectric actuator according to any one of claims 1 to 9, wherein the film (1102, 2402) comprises regions manufactured from different materials.

11. The piezoelectric actuator according to any one of claims 1 to 10, further comprising at least one bending piezoelectric element (1214) attached to the film (1202) within the central region (1212) of the film, which applies a bending moment to the film.

12. The piezoelectric actuator according to any one of claims 1 to 11, wherein the film (1802) comprises a region (1824) that undergoes material modification such that the film has a predetermined flexed shape without the piezoelectric element exerting force on the film.

13. A microfluidic device comprising a piezoelectric actuator (1000) according to any one of claims 1 to 12.

14. The microfluidic device according to claim 13, comprising at least two fluid ports and a valve seat portion (2538) positioned around one of the ports (2534) and protruding from the port toward the membrane (1002), wherein the flexible membrane is operable to open and close the respective fluid paths (2540) by contacting and moving away from the valve seat portion (2538).

15. The microfluidic device according to claim 13, comprising at least two fluid ports, wherein the actuator is operable to discharge fluid from at least one of the ports.

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