Abstract:
An electroactive device comprises at least two layers of material, wherein at least one layer is an electroactive material and wherein at least one layer is of non-uniform thickness. The device can be produced in various sizes, ranging from large structural actuators to microscale or nanoscale devices. The applied voltage to the device in combination with the non-uniform thickness of at least one of the layers (electroactive and/or non-electroactive) controls the contour of the actuated device. The effective electric field is a mathematical function of the local layer thickness. Therefore, the local strain and the local bending/torsion curvature are also a mathematical function of the local thickness. Hence the thinnest portion of the actuator offers the largest bending and/or torsion response. Tailoring of the layer thicknesses can enable complex motions to be achieved.

Description:
CLAIM OF BENEFIT OF PROVISIONAL APPLICATION 
   Pursuant to 35 U.S.C. §119, the benefit of priority from provisional application 60/161,113, with a filing date of Oct. 22, 1999, is claimed for this non-provisional application. 

   ORIGIN OF THE INVENTION 
   The invention described herein was made by an employee of the United States Government and a National Research Council Research Associate and may be used by or for the Government for governmental purposes without the payment of any royalties thereon or therefor. 

   CROSS REFERENCE TO RELATED CASES 
   This application is related to co-pending, commonly owned patent application Ser. No. 09/696,524, filed Oct. 23, 2000, entitled “Polymer-Polymer Bilayer Actuator”, co-pending commonly owned patent application Ser. No. 09/696,528, filed Oct. 23, 2000, entitled “Electrostrictive Graft Elastomers,” and co-pending, commonly owned patent application Ser. No. 09/696,527, filed Oct. 23, 2000, entitled “Membrane Position Control.” 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention is generally related to the field of electroactive actuators. More specifically, it relates to an electroactive actuator having at least one layer of non-uniform thickness. 
   2. Description of the Related Art 
   Actuation devices are used for many applications, including aerospace, fluid flow and biomedical. Space applications include robotics, miniature rovers, and the shaping, tuning, positioning, controlling and deforming of membrane structures. Membrane inflatable and deployable space structures are used by the government and commercially as reflectors, antennas, solar arrays, satellites, solar sails, etc. Although actuation devices are widely used, many challenges exist which limit their performance for high precision applications. Factors affecting precision include surface smoothness, deviation from desired surface profile, surface deformations due to thermal fluctuations, and accurate membrane positioning. Additionally, hydrofoils and airfoils that can optimize their surface shape at varying flow rates are desirable to, for example, increase lift, reduce noise levels, lower vibrations and reduce drag. Other potential uses of actuation devices include precise positioning of display panels and optical index layers. To operate most effectively in the aforementioned applications, actuation devices require sufficient force and strain, and often need to produce complex motions that may include both bending and torsion. 
   Conventional piezoelectric ceramic, polymer, and composite actuators (including piezoelectric, electrostrictive, and electrostatic) lack the combination of sufficient strain and force to most effectively perform the aforementioned functions. Previous concepts for shaping and tuning membrane structures have primarily involved the use of piezoelectric ceramic materials. These ceramic piezoelectrics have the major problems of large mass, high density, low strain and high brittleness. Generally, piezoceramics also need additional mechanical devices to achieve a shaping, tuning, positioning, controlling or deforming function. In contrast to electroceramics, electroactive polymers are emerging as new actuation materials due to their enhanced strain capabilities. 
   Electrostrictive polymer-polymer actuators or other electroactive polymer actuators that provide enhanced strain capabilities can shape, tune, position, control and deform membrane structures, as well as perform in other applications, in ways not previously possible with other materials. An example of such an electrostrictive polymer-polymer actuator is described in the patent application entitled “Polymer-Polymer Bilayer Actuator”, Ser. No. 09/696,524, filed Oct. 23, 2000, hereby incorporated by reference. The greater strain capability provides further possibilities for small-scale applications and integration into skin surfaces. The electroactive actuators can coincide with specific contours to optimize, for example, shapes for fluid flow, reflection and other membrane uses. 
   Existing devices capable of providing complex motion response utilize surface electrode patterning and/or polymer laminates having tailored lamina properties and orientations, such as described in U.S. Pat. No. 4,868,447. It is desirable to obtain complex motion response without requiring tailored surface electroding or laminate design. 
   STATEMENT OF THE INVENTION 
   Accordingly, an object of the present invention is to provide an electroactive device having controlled local strain and curvature. 
   Another object is to provide an electroactive device having a response contour which varies across the device. 
   Another object is to provide an electroactive device that can produce complex motions. 
   A further object is to provide an electroactive device with enhanced strain capabilities. 
   Additional objects and advantages of the present invention are apparent from the drawings and specification that follow. 
   SUMMARY OF THE INVENTION 
   In accordance with the present invention, the foregoing and other objects and advantages are attained by providing an electroactive device having at least two layers of material, wherein at least one layer is an electroactive material and wherein at least one layer is of non-uniform thickness. The device can be produced in various sizes, ranging from large structural actuators to microscale or nanoscale devices. The applied voltage to the device in combination with the non-uniform thickness of at least one of the layers (electroactive and/or non-electroactive) controls the contour of the actuated device. The effective electrical field is a mathematical function (E=V/D, where E is electrical field, V is voltage and D is thickness) of the local layer thickness. Therefore, the local strain and the local bending/torsion curvature are also a mathematical function of the local thickness. Hence the thinnest portion of the actuator offers the largest bending and/or torsion response. Tailoring of the layer thicknesses can enable complex motions to be achieved. 
   In a preferred embodiment, one or more electroactive layers of non-uniform thickness control the curvature of the device. The most responsive portions of the device will be at the thinnest portions of the electroactive layers, where the highest electric fields result. In other embodiments, the curvature can be controlled by varying the thickness of the non-electroactive layer or by varying the thickness of both the electroactive layer(s) and non-electroactive layer. 
   The electroactive device described herein will provide enabling technology to allow variable contouring of the device to expand electroactive actuator use in applications such as motion control, position control, tension control, curvature control, biomedical pulse control, surface flow dynamic control, display panels, optical alignment, optical filters, micro-electromechanical systems, and nano-electromechanical systems. More specifically, it can be utilized in membrane inflatable and deployable structures, and be used for shaping surfaces such as hydrofoils and airfoils to optimize shape for different flow rates. Furthermore, the device could serve to provide precise positioning of an optical index layer for a liquid crystal display and provide positioning control of display panels to reduce glare. 
   Advantages of using polymers for the electroactive layer(s) include low weight, unified materials-device body, simple operation, long lifetime, flexibility, toughness, and ease of processing. However, use of layers (electroactive and/or non-electroactive) of non-uniform thickness to control the curvature can be applied to any materials that can cooperatively produce a sufficient force and strain combination for particular shaping, tuning, positioning, controlling and deforming applications. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A more complete appreciation of the invention and the many of the attendant advantages thereof will be readily attained as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein: 
       FIG. 1A  illustrates a side view of an embodiment of a non-uniform thickness actuator, showing the most responsive portions located at the thinnest points of the active layer closer to the free end. 
       FIG. 1B  illustrates a side view of an embodiment of a non-uniform thickness actuator, showing the most responsive portions located at the thinnest points of the active layer closer to the cantilevered end. 
       FIG. 2  illustrates a side view of a non-uniform thickness actuator fixed at one end, with the thickness of the active layer decreasing towards the fixed ends. 
       FIGS. 3A–3C  illustrate a cross section of a typical hydrofoil or airfoil with a non-uniform thickness actuator, in actuated and non-actuated configurations, attached to the surface of the foil. 
       FIG. 3D  illustrates a cross section of a typical hydrofoil or airfoil with a non-uniform thickness actuator integrated into the foil. 
       FIG. 4  illustrates an embodiment of a non-uniform thickness actuator having stacked electroactive layers, wherein the stacks on either side of the bond interface are alternately activated. 
       FIG. 5  illustrates an embodiment of a non-uniform thickness actuator having multiple electroactive layers. 
       FIG. 6  illustrates thickness variation of a single layer of an actuator. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Referring now to the drawings, and more particularly to  FIGS. 1A and 1B , an electroactive device according to the present invention is shown and referenced generally by the numeral  100 . Electroactive layer  112  is of non-uniform thickness and is bonded to non-electroactive layer  114 , which has uniform thickness. A layer should be understood to be a sheet, strip, film, plate, or the like, which may have various configurations such as planar, annular, and spiral. Although either or both layers can be of nonuniform thickness, nonuniformity of the electroactive layer thickness will produce the greatest strain, and hence displacement capability of the device. Electroactive layer  112  can be any material that responds to electrical activation, including a polymer, ceramic or composite, and is selected based upon the response desired. A preferred material is the electrostrictive graft elastomer described and claimed in “Electrostrictive Graft Elastomer”, Ser. No. 09/696,527, filed Oct. 23, 2000, hereby incorporated by reference. Another preferred embodiment is the polymer-polymer actuator described and claimed in “Polymer-Polymer Bilayer Actuator”, Ser. No. 09/696,524, filed Oct. 23, 2000, also hereby incorporated by reference, wherein the active polymeric web has non-uniform thickness. Non-electroactive layer  114  must have a mechanical modulus sufficient to obtain the desired response in conjunction with electroactive layer  112 . For equal thickness of the electroactive layer  112  and non-electroactive layer  114 , the mechanical modulus of the non-electroactive layer  114  is preferred to be equal to or lower than the mechanical modulus of the electroactive layer  112  in order to achieve maximum bending displacement. Candidate materials include polymers, ceramics, composites, and metals. 
   The layers  112  and  114  are bonded using chemical, physical, mechanical, or biological bonding means. The preferred bonding means provide ease in processing, minimized thickness, as well as the desired stiffness and durability. Especially preferred is a chemical adhesive that is cast and cured at room temperature. The bonding layer thickness depends on the whole configuration of the device, including the material selections for the electroactive and non-electroactive layers, as well as the device&#39;s displacement and stress induced at the bonding interfaces. The thinnest bonding layer that satisfies the device requirements is preferred. Epoxy resin is a suitable chemical adhesive. 
   Layers  112  and  114  are fixedly mounted at  116  and electrically connected to a drive voltage (not shown). When no voltage is supplied, the device remains in the non-activated position  120 . In  FIG. 1A , when voltage is supplied, electrical signals are supplied across the thickness of layer  112 , and the electroactive response of layer  112  causes device  100  to bend to position  140 . The electrical signals are supplied via one or more electrodes  130  disposed on each of the upper and lower surfaces of layer  112 . These electrodes  130  can be disposed via a single layer across the surface or via multiple or patterned electrodes, depending on the desired response. One example of suitable electrodes  130  are gold electrodes, although any material having significant conductivity (generally greater than 10 5  S/m) and fatigue resistance can be used. A conductive polymer having mechanical elasticity comparable to the electroactive material and good adherence to the electroactive material is preferred for the electrode material. Some examples of suitable electrodes are polypyrrole and polyaniline. The drive voltage is dependent on the number of device layers, as well as on the desired displacement, and can range from several volts to several kV. 
   The most responsive area of device  100  is position  140  at the thinnest portion of electroactive layer  112 . Similarly, in  FIG. 1B , the most responsive area of device  100  is position  150 , at the the thinnest portion of electroactive layer  112 . In other embodiments the non-electroactive layer  114  may be of non-uniform thickness, although lesser displacement of the device would be achieved. 
   The thickness ratio between electroactive layer  112  and non-electroactive layer  114  can be tailored to achieve the desired response. If the electroactive and non-electroactive layers have the same mechanical modulus, then the non-electroactive layer thickness should be less than or equal to that of the electroactive layer thickness. If the moduli differ, the thicknesses are optimized based on the application requirements. The thickness of the layers  112  and  114  depend upon the desired response. For multiple electroactive layers, the thicknesses of the layers, the moduli of the layers, and the material selection is tailored to achieve desired results. 
   Referring to  FIGS. 2A and 2B , another embodiment of the electroactive device according to the present invention is shown and referenced generally by the numeral  200 . Electroactive layer  212  is narrowed at each end and is bonded along its length to non-electroactive layer  214 , which has uniform thickness. Device  200  is fixedly attached at  280  to a structure  235  on which the actuator acts. Furthermore, device  200  can be attached to the support layer  235  by chemical or mechanical means. Electroactive layer  212  is electrically connected to a drive voltage (not shown). When no voltage is supplied, as illustrated in  FIG. 2A , the device  200  remains in its non-activated position. When voltage is supplied, as illustrated in  FIG. 2B , the electroactive response of layer  212  causes device  200  to bend to its activated position. The most responsive areas of the device  200  are at the thinnest portions of layer  212 , nearest ends  280  and  290 . 
     FIGS. 3A through 3C  depict an application in which a non-uniform electroactive device is used to optimize characteristics of a hydrofoil or airfoil. Such optimization may include the formation of traveling waves. Cross-Section  300  represents a typical airfoil or hydrofoil. One or more non-uniform thickness electroactive actuators  310  are affixed to the airfoil or hydrofoil, preferably at the leading edge. In the activated positions  320  and  330 , the actuators form a curvature that alters the flow stream  340 .  FIG. 3B  illustrates the actuator displacement resulting from the actuator being fixed at  350  to the airfoil or hydrofoil.  FIG. 3C  illustrates an actuator displacement resulting from the actuator being fixed at  360  to the airfoil or hydrofoil. In another embodiment, shown in  FIG. 3D , one or more electroactive devices  370  are integrated into the airfoil or hydrofoil; i.e., attached to and recessed within the hydrofoil or airfoil  300 . Again, the electroactive devices are affixed at one end to the airfoil or hydrofoil. This embodiment results in a smooth airfoil/hydrofoil surface when the electroactive device(s)  370  are in their inactivated state. 
     FIG. 4  illustrates an embodiment having multiple electroactive layers  400  through  450 . Electroactive layers  400  through  420  form a first stack  470  and electroactive layers  430  through  450  form a second stack  480 . The first stack  470  and second stack  480  are bonded via bonding layer  460 . First stack  470  and second stack  480  are alternately activated. Although electroactive layers  400  through  450  can be different materials, consistent materials are preferred to obtain greater control of the device. 
     FIG. 5  illustrates an embodiment having three electroactive layers  510  through  530  and a single non-electroactive layer  500 . Such a multiple electroactive layer arrangement may be used to obtain greater output force and greater strain/displacement for a given drive voltage. 
   Referring now to  FIG. 6 , the thickness variation of one or more layers is chosen to achieve a desired contour. The thickness of a layer can vary as any function of length (t=f(1)), any function of width (t=f(w)), or as any function of both length and width (t=f(1,w)). This thickness variation acts in cooperation with and/or enhances the contour that could be achieved by material choice, electrode design, or orientation of layers. 
   Although the drawings illustrate specific configurations, the invention is not limited to such specific configurations. At least one electroactive layer is required and at least one non-uniform thickness layer (electroactive or non-electroactive) is required, but each desired application and its associated desired response (strain and force) will dictate the number of electroactive layers and number of nonuniform layers needed. A non-electroactive layer is not required, such as the embodiment shown in  FIG. 4  illustrates; however, if a non-electroactive layer is used, there should be no more than one. As provided earlier, such non-electroactive layer may be of uniform or non-uniform thickness depending on the desired results. Although the embodiments shown illustrate the electroactive devices being fixed at an end, they may be fixed to another location as desired for a specific application. For example, the electroactive device itself could be the membrane to be deformed, such as a reflector, and have a centrally fixed point. 
   Factors which affect the performance of the present invention include: 1) the non-uniformity in layer thicknesses; 2) electromechanical properties of the electroactive layer, such as electric field induced strain, mechanical modulus, and electromechanical conversion efficiency, as well as output energy/energy density; 3) mechanical properties of the non-electroactive layer, such as mechanical modulus; 4) bonding between the layers, as well as 5) the geometric dimension of each component. For an optimized configuration; 1) the electroactive layer(s) offer maximized electric field induced strain and maximized mechanical modulus, therefore, maximized electromechanical output power/energy; 2) the non-electroactive layer offers mechanical modulus not higher than that of the electroactive layer(s); 3) the bonding between layers offers strength, does not allow any significant sliding effect between the electroactive and non-electroactive layers in the direction parallel to the surfaces, and offers maximized durability under working conditions; 4) the relative dimensions of the electroactive layer(s) and non-electroactive layer are chosen according to the requirements of a particular application, with a relatively thin non-electroactive layer being preferred; 5) the thickness of the bonding material is minimized; and 6) the non-uniform thicknesses of layers are designed to meet desired response requirements. 
   Obviously, numerous additional modifications and variations of the present invention are possible in light of above teachings. It is therefore to be understood that within the scope of the appended claims, the invention may be practiced otherwise than is specifically described herein.