Abstract:
A single crystal piezo (SCP) apparatus and method of forming same. The apparatus is ideally suited for actuator and energy harvesting applications. The apparatus includes an SCP layer bonded to a surface of a flexible metal layer while the metal layer is held flattened within a press or other tool. Once the bonding process is complete, the metal layer, it imparts a compressive strain to the SCP layer bonded thereto. A layer of uniaxial graphite may also be bonded to the SCP layer to eliminate the poison&#39;s ratio tension that would otherwise be created in the SCP layer.

Description:
STATEMENT OF GOVERNMENT RIGHTS  
       [0001] This invention was produced pursuant to U.S. Government Contract no. N66604-99-3-4671 with the Defense Advanced Research Projects Agency (DARPA). The U.S. Government has certain rights in the invention. 
     
    
     
       FIELD OF THE INVENTION  
         [0002]    The present invention relates to piezoelectric devices and methods of manufacturing same, and more particularly to a single crystal piezo (SCP) apparatus manufactured to impart a compressive prestress thereto to significantly harden the apparatus.  
         BACKGROUND OF THE INVENTION  
         [0003]    Conventional ceramic piezo wafers, such as PZT, are often constructed for use in connection with bi-morph or unimorph actuators. To use the piezo wafer as an actuator or possibly as an energy harvesting device, it must be strengthened during manufacture by imparting a compressive prestress thereto. One such means for prestressing a wafer of piezoelectric material in a manner that strengthens it is by bonding the layer of piezoelectric material to a “prestressing layer” using a high temperature polyimide adhesive. The compressive stress in the piezoelectric layer is induced by the difference in coefficient of thermal expansion between the piezoelectric material and both the prestressing layer and the adhesive, both of which have much higher coefficients of thermal expansion than the piezoelectric material.  
           [0004]    Another approach to hardening a piezoelectric material involves sandwiching a piezoelectric layer of material between two fiber composite panels that are under tension in at least one direction of their plane during the bonding process. The composite panels are held under tension until the bond cures, at which point they are released and a compressive stress is induced in the piezoelectric material.  
           [0005]    Until the present time, certain “smart” materials, such as single crystal PZN-PT or PMN-PT, which are known as single crystal piezo (“SCP”) material, have not been used in connection with actuators and/or energy harvesting devices because of the lack of hardness of these materials. This lack of hardness makes SCP material highly prone to fracturing when the SCP material is placed under tension. However, SCP material would otherwise be ideally suited for actuator and energy harvesting applications because of its very high energy density, which is many times that of conventional piezo material.  
           [0006]    It is therefore desired to provide a means for manufacturing an actuator or energy harvesting device incorporating an SCP layer of material which is not prone or susceptible to fracturing when placed in tension. Such a hardened SCP material could then be used in a wide variety of tasks such as aerodynamic flow control and structural energy harvesting applications that would otherwise be impossible because of the tendency of the SCP material to fracture when placed in tension.  
         SUMMARY OF THE INVENTION  
         [0007]    The present invention is directed to a single crystal piezo (SCP) apparatus and method of manufacturing same which makes the apparatus ideally suited for actuator and energy harvesting applications. In one preferred form the actuator includes a steel layer which is formed having a predetermined curvature such that one surface thereof forms a concave surface. A layer of SCP material is bonded to the steel layer while the steel layer is held in a flattened condition by a suitable tool. Once bonding is complete, the force is removed and the steel layer is allowed to flex back into its predetermined curvature. This causes a compressive prestrain to be applied to the SCP material as the steel layer assumes its original, predetermined curvature.  
           [0008]    In one preferred embodiment a layer of uniaxial graphite is also bonded to a surface of the SCP material such that the SCP material is sandwiched between the uniaxial graphite layer and the steel layer. The uniaxial graphite material prevents the axis of the SCP wafer normal to its longitudinal surface in contact with the steel layer from going into tension as the steel layer is allowed to flex back into its predetermined curvature. In effect, the graphite fibers of the uniaxial graphite material serve to keep poison&#39;s ratio induced strain in compression and allow the SCP layer to energize the steel layer. Since the uniaxial graphite is orientated in the nonbending direction, any small stiffness is added in the bending direction.  
           [0009]    The resulting SCP apparatus is ideally suited for actuator and energy harvesting applications due to its very high energy density. The compressive strength imparted to the SCP material of the apparatus prevents breakage as the SCP layer flexes during operation.  
           [0010]    Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples are intended for purposes of illustration only and are not intended to limit the scope of the invention. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]    The present invention will become more fully understood from the detailed description and the accompanying drawings, wherein:  
         [0012]    [0012]FIG. 1 is a side view of an SCP apparatus in accordance with a preferred embodiment of the present invention;  
         [0013]    [0013]FIG. 2 is an exploded side view of the components of the apparatus of FIG. 1 prior to assembly;  
         [0014]    [0014]FIG. 3 is a side view of the components of the apparatus of FIG. 2 being bonded together via a suitable tool such as a press;  
         [0015]    [0015]FIG. 4 is a side view of an alternative preferred embodiment of the present invention;  
         [0016]    [0016]FIG. 5 is a side view of the embodiment of FIG. 4 with the SCP layer in a shortened condition and being held against the flexible substrates; and  
         [0017]    [0017]FIG. 6 is a view of the shape that the flexible substrate forms once the SCP layer is bonded thereto.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0018]    The following description of the preferred embodiment(s) is merely exemplary in nature and is in no way intended to limit the invention, its application, or uses.  
         [0019]    [0019]FIG. 1 is a simplified side view of a single crystal piezo (SCP) apparatus  10  in accordance with a preferred embodiment of the present invention. The SCP apparatus  10  generally includes a layer of single crystal piezo (SCP) material  12  which is bonded to a steel layer or substrate  14  via a suitable adhesive  16 . The steel layer  14  typically comprises spring steel, but other suitable materials are brass, graphite epoxy composite and glass epoxy composite. The adhesive typically comprises epoxy with 5% weight glass beads 0.002 inch (0.0508 mm) in diameter, but again, it will be appreciated that any suitable adhesive can be utilized. In a preferred embodiment a layer of uniaxial graphite  18  is also bonded to the SCP material  12  via an adhesive  20 . Preferably, a quantity of conductive epoxy, for example 10 milligrams, is also placed in the center of the adhesive layer  16  to allow for electrical conduction. Adhesive  20  may similarly comprise conductive epoxy. The SCP apparatus  10  can be used, for example, as an actuator to control aerodynamic flow control surfaces on aircraft and other mobile platforms, or just as readily as a structural energy harvesting apparatus for harvesting energy and generating an electrical output as the apparatus flexes.  
         [0020]    With further reference to FIG. 1, an important feature is the formation of the steel layer  14  with a predetermined curvature that produces a concave surface  14   a . During manufacture of the apparatus  10 , as will be described momentarily, a compressive strain is introduced into the SCP material  12  when the metal layer  14  assumes the curvature shown in FIG. 1. This serves to harden the SCP layer  12  and prevent fracturing. Without the compressive strain imparted to the SCP material  12 , flexing of the steel layer  14  could cause fracturing of the SCP material  12 . As a result, without this characteristic of having an induced compressive strain when the SCP apparatus  10  is at rest (i.e., not experiencing any external force that would place it under tension), the apparatus  10  would not be reliable in applications involving actuators and energy harvesting where repeated flexing of the steel layer  14  would be expected.  
         [0021]    Referring to FIG. 2, the manufacture of the apparatus  10  will now be described. Initially, the metal layer  14  is placed against a work surface  22  of a suitable tool, for example a press having a piston  24 . The metal layer  14  is placed with its concave surface  14   a  facing the SCP material  12 . The adhesive layer  16  is shown placed on the concave surface  14   a  of the metal layer  14 , but it will be appreciated that the adhesive could just as readily be applied to a lower surface  12   a  of the metal layer  12 . Adhesive layer  20  is also shown placed on the SCP material  12 , but again this adhesive could also be placed on a lower surface  18   a  of the uniaxial graphite layer  18 .  
         [0022]    The press  24  is used to apply a sufficient force to the uniaxial graphite layer  18  and the SCP layer  12  to thus flatten the metal layer  14  down against the work surface  22  as shown in FIG. 3. The application of force is maintained for a time, for example one hour to twelve hours, sufficient to allow the adhesive layers  16  and  20  to bond to the surfaces with which they are in contact. This securely bonds the SCP layer  12  to the metal layer  14  and the uniaxial graphite layer  18  to the SCP layer  12 . When the piston  24  is released, the steel  14  flexes or “springs” back into its preformed curvature shown in FIG. 1. As it does this, it imparts a compressive strain into the SCP layer along axis  26  shown in FIG. 3. However, due to the poison&#39;s ratio, the opposite axis (designated by arrow  28 ) of the SCP layer  12  would normally go into tension. The uniaxial graphite layer  18  includes fibers which are arranged horizontally in the drawing of FIG. 3 such that the layer  18  is stiff only in the direction indicated by arrow  28 . Thus, the uniaxial graphite layer  18  serves to keep the poison&#39;s ratio induced strain in compression while still allowing the SCP material  12  to experience a compressive strain.  
         [0023]    The foregoing apparatus and method effectively induces a compressive strain along the axis  26  shown in FIG. 3 once the steel layer is allowed to flex back into its preformed curvature. This compressive strain serves to significantly harden the SCP material  14  and prevent fracturing of same during flexing of the metal layer  14  when the SCP apparatus  10  is used as an actuator or energy harvesting device.  
         [0024]    It will be appreciated that the significant energy density of the SCP layer  12  allows the apparatus  10  to be used in an energy harvesting application, for example, to generate sufficient power to power a radio frequency (RF) transmitter. In an actuator application, the apparatus  10  can be used as a bi-morph or unimorph actuator. The manufacturing process described herein further does not rely on the difference in the coefficient of thermal expansion between various layers of the apparatus, as with certain methods of prestressing piezo materials.  
         [0025]    Referring to FIG. 4, an alternative preferred method in accordance with the present invention is shown. In this method, an SCP layer of material  50  is placed on a flexible, electrically conductive substrate  52 . The substrate  52  has a layer of electrically conductive adhesive  54  thereon. In one preferred implementation, adhesive layer  54  is non-conductive but a microliter of electrically conductive adhesive is placed on the adhesive layer  54  at the center of the substrate  52 .  
         [0026]    The substrate  52  and the SCP material  50  are placed in a suitable tool, such as a press  56 , and a voltage is applied across the SCP material  50  via a pair of electrodes  57  temporarily secured to the SCP layer  50  and to the substrate  52 , as shown in FIG. 5. The applied voltage causes the SCP material  50  to shrink in the direction of arrow  58 . This direction is known in the art as the “3-1 direction”. FIG. 5 illustrates the SCP material  50  in its longitudinally shortened condition. The necessary applied voltage field is typically within the range of about 0.5 megavolts per meter to 2 megavolts per meter.  
         [0027]    The SCP layer  50  and substrate  52  are held within the press for a time sufficient to allow the adhesive  54  to fully set. This typically occurs after a time of between about one minute-12 hours, depending on the specific adhesive used.  
         [0028]    Referring to FIG. 6, when the adhesive  54  has fully set, the voltage is removed from the SCP layer  50 . This causes the SCP layer  50  to expand in length along the axis of arrow  58  to its original length, which causes a compressive stress to be induced in the SCP layer by its attachment to the substrate  52 . The substrate  52  will “bow” or bend slightly as a result of the lengthening of the SCP layer  50 . The compressive stress induced significantly hardens and improves the fracture resistance of the SCP layer  50 .  
         [0029]    The description of the invention is merely exemplary in nature and, thus, variations that do not depart from the gist of the invention are intended to be within the scope of the invention. Such variations are not to be regarded as a departure from the spirit and scope of the invention.