The present invention relates to a piezoelectric actuator and to a microfluidic device, such as a microvalve or micropump.

For decades piezo-membrane actuators have been used to provide precise movements, high forces, and low energy performances. Several configurations such as unimorph, bimorph, stack actuators, etc. are proposed to improve the performance of these actuators, and to increase their life time. Most of these actuators are in the form of multi-layer structures of piezo materials as active components mounted on other materials such as metal, polymer or glass as passive mem branes. Due to their intrinsic electro-mechanical characteristics, there is a strain produced in the piezo material when an external voltage is applied, which results in a bending moment as the strain in the piezo material is restrained by the passive layers. A lot of research is done to improve the performances of these actuators, for example by optimizing the bending moment to stiffness ratio of the multi-layered actuator. Others have tried to optimize the electrical field applied by changing the metallization layers and electrodes of the actuators in various array or spiral shapes.

Figures 29 to 31 depict a conventional piezoelectric actuator 10 in three different operational states. For developing a buckling actuator membrane 12, it is known to sandwich the membrane 12 (also often referred to as“diaphragm”) between two piezoelectric rings 14A, 14B, with the membrane 12 and piezoelectric elements 14A, 14B having a flat structure with a constant thick ness, and the membrane 12 having the same outer dimensions as the outer dimension of the piezo elements 14A, 14B (see also for instance publications [1] to [9]).

Furthermore, publication [10] discloses a piezoelectric actuator which comprises a piezoelectric element for performing expansion/contraction motions in accordance with an electric field condi tion, a seat to which the piezoelectric element is adhered on at least one surface thereof, and a supporting member for supporting the piezoelectric element and the seat, wherein the piezoelec tric element and the seat vibrate up and down in accordance with the expansion/contraction mo tions of the piezoelectric element. The seat is connected to a supporting member through a vibra tion film which is less rigid than this seat. However, the surrounding member is only a passive material to provide the surrounding support, and not an active member to induce and control the bending moments and buckling forces.

Further, publication [11] aims to provide a method for compensating low generating force and little displacement, which are disadvantages of the prior piezoelectric bender actuator. Additionally, the document aims to provide a piezoelectric bender actuator capable of controlling generating force and displacement exactly. According to an embodiment, the piezoelectric bender actuator in cludes: a piezoelectric plate that includes a piezoelectric ceramic in the shape of a bar, and a base plate, which is attached to a lower side of the piezoelectric ceramic and is longer than the piezo electric ceramic; and compressing members which are located on both end portions of the piezo electric plate and can compress the piezoelectric plate in parallel with the lengthwise direction. The piezoelectric plate is buckled in one direction when the compressing members compress the both sides of the piezoelectric plate in absence of an electric field. The piezoelectric plate is buck led in the other direction in the presence of an electric field. However, the compressing member is not a piezoelectric element.

Publication [12] relates to piezoelectric energy harvesting using a nonlinear buckled beam. An energy harvester includes a frame having a base, a first side member affixed to the base, and a second side member affixed to the base and spaced apart from the first side member. A beam is coupled between the first side member of the frame and the second side member of the frame. The beam has a substrate layer with a first end affixed to the first side member of the frame, a second end affixed to the second side member of the frame, a first face, and a second face oppo site to the first face. The substrate is elastically deformable in response to the vibratory force. The beam further includes a first piezoelectric layer joined to the first face of the substrate layer and having a terminal for electrical connection to a load, the first piezoelectric layer comprising at least one piezoelectric patch. The frame described in Fig. 1 of document [12] is only to provide a passive and rigid support to create buckling, and does not have an active role in applying and controlling the bending and buckling.

Finally, publication [13] discloses a piezoelectric micropump in which a pump chamber is isolated by a diaphragm. A piezoelectric element is disposed on a back surface of the diaphragm, and the diaphragm is deformed by bending deformation of the piezoelectric element to change the volume of the pump chamber and transport fluid in the pump chamber. A support member for supporting a back surface of the piezoelectric element is provided so that the support member inhibits bend ing of a peripheral portion of the diaphragm in a reverse direction. The support member thus pre vents the piezoelectric element from being floated. Accordingly, the displacement of the piezoe lectric element can be reliably transmitted as the change in volume of the pump chamber, thereby enhancing the fluid transportation performance. However, member 1d shown in Fig. 8 of document [13] describes a block (support member), and not an active piezoelectric member. Yet several intrinsic aspects of the piezo ceramics seem to limit their application where large de flections are required. One is the natural stiffness of these materials that are often in the form of ceramics with stiffness levels of some tens of GPa, in the same range of Aluminium. Therefore the piezo itself greatly contributes to the undesired bending stiffness of the actuator, which in turn leads to the reduced achieved deflection of the entire structure. In addition, the large stiffness corresponds to the large stresses created in the material even under small strains, and here comes the second intrinsic characteristic of these materials that makes realizing large deflections so chal lenging, and that is the brittle nature of these materials. In other words, even if a configuration could achieve large deflections despite the high bending stiffness of the piezo materials, it would not be able to reach to strains more than some tenths of percent, due to the brittle nature of these materials.

Therefore, there is still a need for an improved piezo-actuation concept, which combines buckling forces with bending moments, resulting in higher deflection and increased lifetime of the piezoe lectric actuator.

This object is solved by the subject matter of the independent claims. Advantageous embodiments of the present invention are the subject matter of the dependent claims.

The present invention is based on the finding that actuators can be developed with a better control by combining the bending and buckling forces, and achieving higher performance in terms of de flection and force/pressure capabilities, if the membrane and the piezoelectric element do not have an identical outline. Instead, the membrane’s dimension is smaller or larger than those of the piezoelectric elements (in the following also sometimes referred to as“piezo element”).

Advantageously, according to the present invention, the actuator is designed so that in operation buckling and bending forces can be combined in both deflection directions of the actuator mem brane. In other words, compressive forces can be generated for both directions of the deflection, in addition to the bending forces.

In particular, a piezoelectric actuator according to the present invention comprises a deflectable actuator membrane, and at least one piezoelectric element operable to perform expansion and contraction motion depending on an electric field applied to the piezoelectric element, wherein the piezoelectric element is attached to a part of the actuator membrane for exerting mechanical force on the membrane, and wherein the piezoelectric element leaves open a central region of the membrane and has a peripheral outline that does not coincide with an outline of the actuator membrane. Firstly, the membrane may have a smaller diameter than the outer diameter of the piezoelectric element. In particular, the piezoelectric element may comprise at least one ring shaped planar plate which is attached to the membrane to cover a ring shaped peripheral region of the membrane with a ring shaped inner region of the piezoelectric element.

The advantages of a smaller membrane dimension can be more easily explained in an energy analysis approach. The energy of the piezo materials results in the deformation energy of the actuator and the external work done by the actuator (such as displacing a liquid). Therefore, if the membrane has a bigger area (such as the ones referenced in [1] to [10]), the deformation energy is consumed in compressing and deforming a bigger area of the membrane (the entire membrane) and the corresponding adhesive which is transferring the stress from the piezo rings to the mem brane. By providing 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. Therefore, as the most important area of the membrane which does the actual mechanical work is the region inside the piezo sections, reducing the membrane size ideally only to this region can considerably increase the performance, besides reducing the material costs.

Alternatively, the membrane’s dimensions may be larger than those of the piezoelectric element. This extension of the membrane beyond the piezoelectric element’s dimensions can be used for mounting and support purposes, creating a longer moment arm compared to the case that the mounting and support area is limited within the piezo dimensions, thus allowing for a larger achiev able deflection. In particular, the piezoelectric element comprises at least one ring shaped planar plate which is attached to the membrane and has an outer diameter that is smaller than an outer diameter of the membrane.

In the context of the present invention,“ring shaped” means a structure which surrounds an empty center with a closed or partially interrupted area having an inner outline and an outer outline. The ring shape in particular is not restricted to circular outlines, but also includes any arbitrary outline, for instance oval, rectangular, or irregular. Furthermore, it should be noted that in the plane of the membrane, neither the membrane nor the piezoelectric element need to be symmetric. The inner and outer outline of the piezoelectric element also do not have to possess central similarity with respect to each other.

Furthermore, the piezoelectric element may comprise a first and a second ring shaped planar plate, the first and the second ring shaped planar plate differing from each other regarding their outer and/or inner diameters, wherein the first and the second ring shaped planar plates are at tached to opposing peripheral surfaces of the membrane. Such a configuration creates asymmet ric forces.

In order to generate asymmetric forces in the two piezoelectric elements, several approaches can be taken into account. Beside the more obvious approach of applying asymmetric voltages to the two piezoelectric elements, the piezoelectric elements’ dimensions can differ from one another to further emphasize the generation of asymmetry towards the desired direction. Creating more ra dial contraction in one of the piezoelectric elements compared to the other one generates a net bending moment in addition to the buckling forces, due to the higher force-multiplied-by-distance to the neutral axis generated by the bigger piezo ring.

To create asymmetric forces in the two piezo rings, the thickness of them can vary, as the force generated in the piezo can be varied by changing the thickness, in other words, the first and the second ring shaped planar plates may have a differing thickness. This is due to two effects. One is due to the change in generated electric field depending on the applied voltage, and the second is due to stress capabilities of the piezo which varies by the thickness. The same electrical fields are applied on the two piezo sections, resulting in a higher force generated in the piezo electric element with the greater thickness, and therefore a resultant bending moment is created together with the net buckling forces.

Such an asymmetry can also be achieved by providing a piezo ring only on one side of the mem brane. Compared to the above mentioned configuration, this configuration results in a more dis tinct bending moment due to lack of opposition by the opposing piezo. A first disadvantage of such an arrangement, however, is the reduction of buckling forces. The other disadvantage is the re duction of bending control, as the bending moments in this configuration are limited to those gen erated by the buckling forces (or tension forces of piezos in case they are activated to move in the opposite direction than the inward buckling direction), considering that these forces are applied with a distance from the structures neutral axis. It should be noted that this particular cross section is not limited to circular actuators. The same concept can be used for other shapes such as rec tangular and oval or even nonhomogeneous or arbitrary forms which can result in the same buck ling effect.

Advantageously, the membrane may comprise multiple sections each from a different material, or even empty regions. For example, it can have a material with high stiffness (Young’s modulus) in the central part (to have a higher force and pressure capabilities), and another more flexible ma terial in the outer part. This allows for a more flexible applied boundary condition and therefore an achieved non-zero slope at the position of mounting and support, which leads to a higher final deflection. Furthermore, a void between the first and the second ring shaped planar plate is at least partly filled with a preferably flexible spacer material.

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

According to a further advantageous embodiment of the present invention, the piezoelectric ele ment comprises at least one ring shaped planar plate which is attached to the membrane abut- tingly at an outer circumferential end face of the membrane. In other words, two essentially planar buckling piezoelectric elements can be replaced by a single piece (for example in case of a circular cross section, two piezo rings can be replaced with one piezo tube), which applies pure tension or compression (buckling) to the structure, thus amplifying the deflection and force produced by a bending piezo which may be located in the central part of the membrane

When the piezoelectric element is attached to only a part of the outer circumferential end face of the membrane, this buckling piezo (such as a tube) is placed asymmetrically with respect to the neutral axis of the membrane, and thus produces a combination of buckling together with a bend ing moment in the membrane.

Furthermore, the membrane may comprise regions with material modifications, so that the mem brane has a pre-defined deflected shape without the piezoelectric element exerting a force on the membrane. According to an advantageous embodiment of the present invention, a pre-buckling in the membrane can be created by applying local changes to the material of the membrane, such as laser treating the surface, which can result in various range of physical or chemical changes such as thermal expansion, oxidation, material/alloy phase change, etc. This results in different effects, such as local volume change, swelling or concentrated pre-stress in the material which then can result in a pre-stress/pre-strain state which also may cause a pre-buckling in a mem brane. The amount of pre-buckling can be changed/tuned by various lasering parameters (for example pulse rate, frequency, power, etc.), geometrical parameters (places to target, pattern, number of repetitions and so on), thermal and other parameters.

Furthermore, the membrane may have a thickness that varies along a radial direction of the mem brane, and/or may comprise regions fabricated from different materials. In order to provide mechanical support, the piezoelectric element may further comprise a support element for mechanically fixing the membrane.

According to an advantageous embodiment, the piezoelectric actuator may further comprise at least one bending piezoelectric element which is attached to the membrane in a central region of the membrane for applying a bending moment on the membrane. Such a bending piezo can be added in the central part of the membrane to combine the“traditional bending configuration” with the buckling approach. Extra bending moment applied by the“bending piezo” (whether by expansion or contraction) will increase the force and pressure carrying capabilities of the actuator.

The bending piezo can also eliminate the need for a double-buckling-piezo configuration by provid ing the required bending moment to control the actuator towards the desired direction (by con tracting or expanding). The buckling forces from the side then just have amplifying effects on the force and deflection.

There can be several bending piezoelectric elements in combination to one or more buckling ones, for example in different locations of the membrane to create more complicated shapes, or on both sides of the membrane to create more bending moment. The bending moment can be increased in this case which for example simplifies the flow control of a proportional valve, especially as it reduces the bifurcation of buckling, when the membrane is close to its midway (initial) position.

For mounting the actuator membrane, flexible support means can be provided advantageously. Such a flexible mounting instead of a fixed clamping of the membrane allows for a non-zero slope in the peripheral region of the membrane. This leads to a higher final deflection of the membrane. For instance, a pair of elastic O-rings can be used to support the membrane. However, also other suitable flexible mounting means can be used.

The present invention also relates to a microfluidic device comprising a piezoelectric actuator ac cording to the present invention. For instance, the microfluidic device may be a microvalve having at least two fluid ports and a valve seat, which is arranged around one of the ports and protrudes from the port towards the membrane, so that the deflectable membrane is operable to close and open the fluidic pathway, respectively, by coming in contact with the valve seat and getting away from it. The present invention further provides a microfluidic device operable as a micropump, wherein the moving actuator pushes the fluid out of at least one of the ports.

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

However, all references to the micropump and microvalve applications are only to clarify the ad vantages of tuning the performance, and are not to limit the application to these fields. Other ap plications can be adaptive optics (such as adaptive lenses and adaptive mirrors), precision move ment, MEMS acoustics (such as microphones and loudspeakers), etc.

With“rigid” as used in this application is meant stiff, unyielding, i.e. a rigid structure is not adapted to be deformable during normal use of the structure.

With“flexible” as used in this application is meant non-stiff, non-rigid, i.e. the material can undergo elastic deformation, regaining its original shape as long as it has not entered into the plastic region.

With“stretchable” as used in this application is meant resilient, i.e. elastically deformable with elongation. A stretchable structure is adapted to be elastically deformed during normal use (with elongation).

With“compressible” as used in this application is meant resilient, i.e. elastically deformable with a reduction of size in the direction of an applied pressure. A compressible structure is adapted to be elastically deformed during normal use (with reduction of dimension).

The accompanying drawings are incorporated into and form a part of the specification to illustrate embodiments of the present invention. These drawings together with a description serve to explain the principles of the invention. The drawings are merely for the purpose of illustrating the preferred and alternative examples of how the invention can be made and used and are not to be construed as limiting the invention to only the illustrated and described embodiments. Furthermore, several aspects of the described embodiments may form - individually or in different combinations - solu tions according to the present invention. Further features and advantages will become apparent from the following more particular description of the various embodiments of the invention, as illustrated in the accompanying drawings, in which like references refer to like elements, and wherein:

Fig. 1 shows a schematic cross sectional representation of a piezoelectric actuator according to a first aspect;

Fig. 2 shows a schematic cross sectional representation of a piezoelectric actuator according to a second aspect; Fig. 3 shows a schematic cross sectional representation of a piezoelectric actuator according to the first aspect having a first membrane configuration with varying thickness;

Fig. 4 shows a schematic cross sectional representation of a piezoelectric actuator according to the first aspect having a second membrane configuration with varying thickness;

Fig. 5 shows a schematic cross sectional representation of a piezoelectric actuator according to a further aspect;

Fig. 6 shows a schematic cross sectional representation of a piezoelectric actuator according to a further aspect;

Fig. 7 shows a schematic cross sectional representation of a piezoelectric actuator according to a further aspect;

Fig. 8 shows a schematic top view representation of a piezoelectric actuator according to a further aspect;

Fig. 9 shows a schematic cross sectional representation of a piezoelectric actuator according to a further aspect;

Fig. 10 shows a schematic cross sectional representation of a piezoelectric actuator according to a further aspect;

Fig. 11 shows a schematic cross sectional representation of a piezoelectric actuator according to a further aspect;

Fig. 12 shows a schematic cross sectional representation of a piezoelectric actuator according to a further aspect;

Fig. 13 shows a schematic cross sectional representation of a piezoelectric actuator according to a further aspect;

Fig. 14 shows a schematic cross sectional representation of a piezoelectric actuator according to a further aspect;

Fig. 15 shows a schematic cross sectional representation of a piezoelectric actuator according to a further aspect in a deflected position; Fig. 16 shows a schematic cross sectional representation of the piezoelectric actuator of Fig. 15 in a neutral position;

Fig. 17 shows a schematic cross sectional representation of a piezoelectric actuator according to a further aspect;

Fig. 18 shows a schematic cross sectional representation of a membrane according to a fur ther aspect;

Fig. 19 shows a schematic cross sectional representation of a membrane according to a fur ther aspect;

Fig. 20 shows a schematic cross sectional representation of a membrane according to a fur ther aspect;

Fig. 21 shows a schematic cross sectional representation of a membrane according to a fur ther aspect;

Fig. 22 shows a schematic cross sectional representation of a piezoelectric actuator according to a further aspect;

Fig. 23 shows a schematic cross sectional representation of a piezoelectric actuator according to a further aspect;

Fig. 24 shows a schematic cross sectional representation of a piezoelectric actuator according to a further aspect;

Fig. 25 shows a schematic cross sectional representation of a microvalve according to an ad vantageous aspect in a first operational state;

Fig. 26 shows a schematic cross sectional representation of the microvalve of Fig. 25 in a second operational state;

Fig. 27 shows a schematic cross sectional representation of the microvalve of Fig. 25 in a further operational state;

Fig. 28 shows a schematic cross sectional representation of the microvalve of Fig. 25 in a further operational state; Fig. 29-31 show schematic cross sectional representations of a known piezoelectric actuator in different operational states.

Referring now to the drawings and in particular first to Fig. 1 , the present invention will be explained in more detail.

Fig. 1 shows a schematic cross sectional representation of a piezoelectric actuator 100 according to a first exemplary aspect of the present invention the piezoelectric actuator 100 comprises a deflectable membrane 102 and two ring shaped piezoelectric elements 104A, 104B. As indicated by the arrows 106, the piezoelectric elements 104 exert an inwardly directed force when an electric field is applied to the piezoelectric elements. It has to be noted, that throughout the present dis closure the electrodes of the piezoelectric elements are shaped according to their polarization as this is known to a person skilled in the art to cause the piezoelectric element to provide inward or outward forces (compression and tension), but are not shown in the drawings in order to avoid obscuring the general ideas.

Furthermore, the piezoelectric elements may be mechanically connected to the membrane in any suitable manner, for instance by means of an adhesive layer or the like.

According to the present invention, the membrane 102 has a smaller dimensions than the piezo electric elements 104. In other words, the piezoelectric elements are in mechanical contact with the membrane in the region which is much smaller than the surface of the piezoelectric elements. For instance, the piezoelectric elements 104A, 104B may be attached to the membrane 102 with a proportion of their overall area of around 20%. However, other proportions are of course possible and it has to be noted that all the Figures are not drawn to scale regarding distances as well as thicknesses.

By combining the bending and buckling of the membrane 102 for instance in the same manner as shown in Figures 29 to 31 , a better control is possible and a higher performance can be achieved in terms of deflection and force and/or pressure capabilities.

The advantages of a smaller membrane dimension can be more easily explained in an energy analysis approach. The energy of the piezo materials results in the deformation energy of the actuator and the external work done by the actuator (such as displacing a liquid). Therefore, if the membrane has a bigger area, the deformation energy is consumed in compressing and deforming a bigger area of the membrane 102, namely the entire membrane, and the corresponding adhe sive which is transferring the stress from the piezo rings to the membrane. In contrast thereto, with a smaller membrane area (such as the one shown in Fig. 1), the deformation energy is used more efficiently in the central region of the membrane 102, resulting in more deflection and achieving more external work. Therefore, as the most important area of the membrane which does the actual mechanical work is the region inside the piezo sections, reducing the membrane size ideally only to this region can considerably increase the performance, beside reducing the material costs.

Moreover, the membrane 102 can have an initial curvature, for example due to a pre-stress such as pre-bending or pre-buckling, to pronounce an asymmetric deflection/force in the desired direc tion. Furthermore, this curvature will reduce the required force to achieve a certain deflection.

On the other hand, as shown in Fig. 2, a piezoelectric actuator 200 may comprise a membrane 202 which is larger in diameter than the piezoelectric elements 204. This extension of the mem brane beyond the piezoelectric elements’ dimensions can be used for mounting/support purposes (as shown in Fig. 2), creating a longer moment arm (compared to the case that the mounting/sup port area is limited within the piezo dimensions) thus allowing for a larger achieved deflection. In Fig. 2 support elements 208 are schematically shown as O-rings. It is clear for a person skilled in the art that any suitable form of support element 208 may of course also be used.

A further advantageous variation of the piezoelectric actuator 300 is shown in Fig. 3. Here, the membrane 302 has a thickness that is reduced in the center compared to the peripheral region. On the other hand Fig. 4 shows a piezoelectric actuator 400 with a membrane 402 that is thicker in the center compared to the marginal region, which is in contact with the piezoelectric elements 404.

As the thickness of the membrane contributes with the third power to the flexural rigidity, the in creased thickness in the center (such as the example shown in Fig. 4) results in a higher trans ferred moment through the membrane and thus higher working pressure, i. e. the actuator 400 can deflect against higher pressures and/or forces. The disadvantage will be a reduced deflection. This increase in thickness can also for example be implemented by adding/mounting other layers onto the membrane 402, from the same material or different materials.

It should be noted that, although the increased thickness can result in a reduced deflection, de pending on the design parameters it can result in a higher volume displacement.

Reduction of the membranes thickness in the center of the membrane 302 results in higher achieved deflections due to the reduced flexural rigidity of the membrane, in the cost of reducing the working pressure and generated force. The tuning between higher achieved deflections or higher achieved pressure/force can give the designer the option to optimize the desired characteristic in every application. For example in micropump applications, the higher deflection of the actuator can result in higher generated flowrate. In microvalve applications, having a higher achieved deflection in the actuator enables the valve to achieve higher flow factors.

On the other hand, higher pressure and force capability of the actuator can result in higher achieved backpressure in micropump applications, or can result in higher working pressure ranges in microvalve applications.

In order to generate asymmetric forces in the two piezoelectric elements to emphasize the gener ated deflection/force more towards one direction, several approaches can be used. Beside the more obvious approach of applying asymmetric voltages to the two piezoelectric elements, the piezoelectric elements’ dimensions can differ from one another to further emphasize the genera tion of asymmetry towards the desired direction. Creating more radial contraction in one of the piezoelectric elements compared to the other one (as shown with black arrows) generates a net bending moment (shown with arrows 510 in Fig. 5) beside the buckling forces, due to the higher force-multiplied-by-distance to the neutral axis generated by the bigger ring shaped piezoelectric element 504B.

A further example of a piezoelectric actuator 600 with asymmetric forces generated by the piezo electric elements 604 is shown in Fig. 6.

According to this example, the thickness of two opposing piezoelectric elements 604A, 604B dif fers, while the diameters of the piezoelectric elements 604A, 604B are equal. It should be noted that of course also additionally differing diameters can be provided. By using piezoelectric ele ments 604A, 604B with different thicknesses, asymmetric forces are generated due to two effects. Firstly, the electric field generated by a given applied voltage changes. Secondly, the stress char acteristics of the piezoelectric elements 604A, 604B varies depending on the thickness. Fig. 6 schematically illustrates this effect. When the same electrical field is applied to the piezoelectric elements 604A, 604B, higher forces 606B are generated in the thicker piezoelectric element 604B. Thus, a resulting bending moment 610 is generated together with the net buckling forces.

A further advantageous example of a piezoelectric actuator 700 according to the present invention is shown in Fig. 7. As shown in this Figure, an asymmetry of the actuating forces can be achieved by providing a ring shaped piezoelectric element 704 only on one side of the membrane. Compared to the previously described configuration, this configuration results in creating a higher bending moment 710 due to the lack of opposition by another piezoelectric element. However, the configuration shown in Figure 7 has the disadvantage that the generated buckling forces are lower. Another disadvantage is the reduction of bending control because in this configuration, the bending moments 710 are limited to those generated by the buckling forces (or tension forces of the piezoelectric element 704, in case they are activated to move in the opposite direction than the inward buckling direc tion), considering that these forces are applied with a distance from the structure’s neutral axis.

As illustrated schematically as a top view in Fig. 8, a piezoelectric actuator 800 according to the principles of the present invention is of course not limited to circular shapes. The same concept can be used for other shapes, such as rectangular and oval or even nonhomogeneous or arbitrary forms which can result in the same buckling effect, as shown in Fig. 8.

According to further advantageous examples of the present disclosure, the membrane may com prise multiple sections each comprising a different material, or even voids. As shown in Fig. 9, for instance, a material with a high stiffness, i. e. a high Young’s modulus, can be provided in a central region 912 of the membrane 902, in order to achieve higher force and pressure capabilities. An other more flexible material may be used in the peripheral region 914. This allows for a more flexible applied boundary condition at the periphery of the membrane 902. Accordingly, a non-zero slope can be achieved at a position where the mounting means (the support means) 908 is lo cated. This leads to a higher final deflection of the membrane.

Alternatively, as shown in Fig. 10, a preferably flexible spacer 1016 may be provided between the piezoelectric elements 1004A, 1004B to create a more flexible applied boundary condition at the periphery of the membrane 1002. According to this example, the support means 1008 is provided within the region of the piezoelectric elements 1004A, 1004B. The area between the piezoelectric elements 1004A, 1004B is filled with the spacer 1016 in order to act as a more flexible support without hindering the transfer of inwardly directed forces of the piezoelectric elements 1004A, 1004B, which have to be transferred to the central part of the membrane 1002.

A further advantageous example of a piezoelectric actuator 1100 is shown in Figure 1 1. According to this example, the material properties of the membrane 1 102 vary for instance in a radial direc tion. The material properties may be changed by further treatments, for instance with thermal treatment such as annealing and sintering, with laser aberration, with alloy phase change or the like. Fig. 11 shows one example where the central part 1112 of the membrane 1102 is hardened. In case of the application in a microvalve, this hardening of the central region 11 12 can increase the established leak-tight area between the central part of the membrane 1102 and a valve seat, which increases the dynamic working pressure of the valve.

As illustrated in Fig. 1 1 , the piezoelectric elements 1 104A, 1 104B exert different radial forces 1106A, 1106B on the membrane 1 102. Thus, bending forces 11 10 are generated in addition to the mere buckling forces.

Figures 12 to 16 illustrate a further advantageous embodiment of the present invention where at least one bending piezoelectric element is provided in addition to the piezoelectric elements that generate buckling forces. By adding such a bending piezoelectric element to the central region of the membrane, the traditional bending configuration can be combined with the more recent buck ling approach. An extra bending moment is applied by the bending piezoelectric element by ex pansion or contraction.

As shown in Fig. 12, one such bending piezoelectric element 1214 is attached to a central region 1212 of the membrane 1202. Consequently, the membrane 1202 of the piezoelectric actuator 1200 is actuated by two piezoelectric elements 1204A, 1204B in a radial direction and is addition ally bent by the contraction and expansion (symbolized by the double arrows 1216) of the bending piezoelectric element 1214. Thus, the force and pressure carrying capabilities of the piezoelectric actuator 1200 are improved.

As shown in Fig. 13, attaching a bending piezoelectric element 1314 to a central region 1312 of the membrane 1302 may even eliminate the necessity of providing two opposing piezoelectric elements arranged on both sides of the membrane. Instead, only one piezoelectric element 1304 is arranged on one surface of the membrane 1303. In this case, the bending moment 1310 which is needed to control the actuator 1300 towards a desired direction is created by expansion or contraction of the bending piezoelectric element 1314. In Fig. 13, for instance, expansion of the bending piezoelectric element 1314 will cause the membrane 1302 to bend upwards. The buckling forces exerted from the side towards the center just have amplifying effects on the force and re sultant deflection.

According to a further aspect of the present disclosure, which is shown Fig. 14, the pair of opposing buckling piezoelectric elements is replaced by one single piezoelectric element 1404. For in stance, a tube shaped piezoelectric element 1406 may be provided for causing the radially ori ented buckling forces 1406 on the membrane 1402. This tube shaped piezoelectric element 1404 interacts with a circumferential face 1418 of the membrane 1402 instead of a upper or lower sur face 1420, 1422 as this was shown for the previous examples.

Thus, the piezoelectric element 1404 applies pure tension or compression (buckling) forces to the membrane 1402, so that the force and deflection produced by the bending piezoelectric element 1414, which is located in a central region 1412 of the membrane 1402, is amplified.

As further shown for the piezoelectric actuator 1500 depicted in Figs. 15 and 16, there can be several bending piezoelectric elements 1514 in combination with buckling piezoelectric elements 1504 (or 1204), for example in different locations of the membrane 1502 to create more compli cated shapes, or on both sides of the membrane 1502 (such as shown in Figures 15 and 16) to create more bending moment. The bending moment can be increased in this case, which for ex ample simplifies the flow control of a proportional valve, especially as it reduces the bifurcation of buckling, when the membrane is close to its midway (initial) position. Figures 15 and 16 show an example of this configuration in two operational states (the deformed and neutral state, respec tively). The bending piezoelectric elements 1514A, 1514B are in this example centrally and sym metrically located on the membrane 1502. The same direction of bending is applied by one 1514A of the bending piezoelectric elements expanding, and the other one 1514B contracting.

Figure 17 shows a further advantageous example of a piezoelectric actuator 1700. According to this example, only one buckling piezoelectric element 1704 (such as a tube) is placed asymmet rically with respect to the neutral axis of the membrane 1702 to produce the combination of buck ling forces 1706 together with a bending moment 1710 in the membrane 1702.

As will be explained with reference to Figures 18 to 21 , a pre-buckling in the membrane 1802 can be achieved by generating local changes of the material characteristics.

For instance, a laser treatment of the membrane’s surface is performed to cause various range of physical or chemical changes such as thermal expansion, oxidation, material and/or alloy phase change, etc. This results in different effects such as local volume changes, swelling, or concen trated pre-stress in the material. These effects can be used to generate a pre-stress or pre-strain state which also causes a pre-deflection, such as a pre-buckling in the membrane 1802. Fig. 18 for example shows an initially flat membrane 1802 with initial state of the regions 1824 which are supposed to be affected by special treatment such as lasering.

These areas 1824 are then expanded as shown in Figure 19, creating a radial force which due to its distance from the neutral axis 1826 also results in a bending moment 1810 around this axis. The result is a membrane 1802 which is deformed in the form of pre-buckling and/or pre-bending. The amount of pre-buckling can be changed and tuned by various lasering parameters (for exam ple pulse rate, frequency, power, etc.), geometrical parameters (places to target, pattern, number of repetition, etc.), thermal and other parameters.

Furthermore, exposing the other side of the membrane as well can increase the buckling force 1806, but reduce the bending moment 1810, as shown in Fig. 20. Therefore, different magnitudes as well as different states of pre-buckling can be achieved. The membrane 1802 can be treated to be mono-stable (only having one stable equilibrium on one side), or bi-stable (having two stable equilibrium states i.e. on both sides). Even more complicated shapes or other modes of pre-buck- ling can also be realized with more complicated combinations such as different laser patterns as shown schematically in Fig. 21.

As shown for the piezoelectric actuators 2200, 2300 depicted in Figures 22 and 23, the membrane 2202, 2302 may be formed to have an initial curvature due to a pre-stress, such as pre-bending, pre-buckling or a material treatment as shown in Figures 18 to 21. Such an initial curvature pro nounces an asymmetric deflection and force in the desired direction. Furthermore, this curvature reduces the required force to achieve a certain deflection in operation.

Fig. 24 shows a further advantageous aspect of a piezoelectric actuator 2400 according to the present invention. According to this example, the membrane 2402 comprises a central portion 2412 which is hardened in order to enhance the forces that can be exerted for instance against a valve seat.

As an exemplary application of a piezoelectric actuator, Figures 25 to 28 illustrate a fluidic control device 2500 comprising a piezoelectric actuator 1000 as shown in Figure 10. According to 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 means of two elastic O-rings 1008 held in notches 2532. The base 2528 comprises an inlet 2534 and an outlet 2536. A valve seat 2538 surrounds the inlet 2534 and the membrane 1002 is arranged close to the valve seat 2538, so that with a downwardly oriented displacement of the membrane 1002 same interacts with the valve seat to close the fluidic path from the inlet 2534.

Fig 25 shows the fluidic control device 2500 in its initial state. By contracting one of the piezoelec tric elements and expanding the other one, the actuator 1000 bends toward the expanded piezo electric element, as shown in Fig 26, in which the actuator bends towards the valve seat 2538. Advantageously, the flexible support by means of the O-rings 1008 allows a non-zero slope of the actuator and thus the membrane, as can be seen from this Figure. Instead of the fixed boundary condition of a firmly clamped cantilever, this allows for a higher final deflection of the membrane. The O-rings serve as a hinge and at the same time as a sealing material. Of course, also other flexible support means, for instance laminated silicone layers, may also be employed instead of a pair of O-rings shown in Figures 25 to 28.

The actuator 1000 can buckle in the already bent direction, if both piezoelectric elements contract, as shown in Fig 27. In the similar manner, the actuator 1000 can bend and buckle away from the valve seat 2538 to open the fluidic connection 2540 between the inlet 2534 and outlet 2536 ports, as shown in Fig 28. As the gap between the membrane 1002 and valve seat 2538 is a key factor in defining the flow through of a valve, by controlling the deflection of the actuator and therefore the resulting gap, a proportional valve can be realized which controls the flowrate and/or pressure of the fluid.

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

Moreover, the valve seat may be formed as one integral part with the bottom plate, thus being a hard valve seat.

In summary, it has to be noted that the piezoelectric elements according to the present disclosure do not necessarily have a uniform thickness and may have a non-planar shape.

No limitations are intended regarding the adhesion between the piezoelectric elements and the various membranes.

No limitations are intended with respect to the polarization direction, the electrode structure of the piezoelectric elements, and the applied voltage directions. The only important point is that, regard less of the polarization and applied voltage, axially compressive buckling forces are generated.

Furthermore, each piezoelectric element may comprise an array of piezoelectric elements or be fabricated in the form of a stack, to reduce the applied voltage while maintaining the same electric field, or to increase the applied force.

All the above references to the micropump and microvalve applications are only to clarify the ad vantages of tuning the performance, and are not intended to limit the application to these fields. Other applications can be adaptive optics, precision movement, MEMS acoustics (such as MEMS loudspeakers), and the like.

References:

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[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 Interna tional 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,” Mi croelectronic 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 micros copy 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] A. Shabanian, F. Goldschmidtboeing, C. Dhananjaya, H. Bettaswamy Gowda, E. Bae- umker and P. Woias, Ed., Low fluidic resistance valves utilizing buckling actuators. [Frankfurt am Main]: VDE VERLAG GMBH, 2017.

[10] CN101336562B

[1 1] KR20140128273 A

[12] WO2018035542A 1

[13] US8454327B2