Patent Publication Number: US-2006016055-A1

Title: Piezoelectric composite apparatus and a method for fabricating the same

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
      This application is a divisional of pending U.S. patent application Ser. No. 10/653824, filed Sep. 3, 2003. 
    
    
     ORIGIN OF THE INVENTION  
      The invention described herein was made by employees of the United States Government and may be used by or for the Government for governmental purposes without the payment of any royalties thereon or therefor. 
    
    
     BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      The present invention is generally related to piezoelectric fiber composite strain actuators.  
      2. Description of the Related Art  
      Conventional piezoelectric fiber composite actuators are typically manufactured using a layer of extruded piezoelectric fibers encased in protective polymer matrix material. Interdigitated electrodes etched or deposited onto polymer film layers are placed on the top and bottom of the fibers to form a relatively thin actuator laminate. Protecting the fibers in a matrix polymer strengthens and protects the piezoelectric material. The resulting package is more flexible and conformable than actuators formed from monolithic piezoelectric wafers. These actuators can be easily embedded within or placed upon non-planar structures using conventional manufacturing techniques. In addition, the use of interdigitated electrode poling permits production of relatively large, directional in-plane actuation strains. The directional nature of this actuation is particularly useful for inducing shear (twisting) deformations in structures.  
      Unfortunately, the methods of manufacturing conventional piezoelectric fiber composites typically use relatively high cost, extruded, round piezoelectric fibers. Moreover, alternative methods of manufacture using square fibers, which are milled from lower cost monolithic piezoelectric wafers, have been unsuccessful due to the difficulty of aligning individual square fibers during actuator assembly without shifting and rolling. Rolled square fibers tend to expose sharp corners and edges which can sever the interdigitated electrode layers during the final process of actuator assembly. Both the round and square fiber approaches require individual handling of piezoelectric fibers during assembly, thereby resulting in relatively high manufacturing costs.  
      Another disadvantage of conventional piezoelectric fiber composite actuators is the requirement of relatively high operating voltages. High operating voltages are needed to produce electric fields which are sufficiently strong to propagate through the protective polymer material encasing the piezoelectric fibers. These electrode voltages are several times higher than those theoretically required to produce a given strain in the unprotected piezoelectric material. Additionally, round fibers have a low contact area with the electrode, thereby causing losses and decreased efficiency. To compensate for these losses, increased voltages are required. Conventional techniques for applying electrodes directly in contact with the piezoelectric fibers have thus far not been practical.  
      It is therefore an object of the present invention to provide an improved piezoelectric fiber composite strain actuator and a method for making same.  
      Still other objects and advantages of the present invention will in part be obvious and will in part be apparent from the specification.  
     SUMMARY OF THE INVENTION  
      The above and other objects and advantages, which will be apparent to one of skill in the art, are achieved in the present invention which is directed to, in one aspect, a method for fabricating a piezoelectric macro-fiber composite actuator. The first step comprises providing a structure comprising piezo-electric material which has a first side and a second side. First and second films are then adhesively bonded to the first and second sides, respectively, of the piezo-electric material. The first film has first and second conductive patterns formed thereon which are electrically isolated from one another and in electrical contact with the piezo-electric material. In one embodiment, the second film does not have any conductive patterns. The first and second conductive patterns of the first film each have a plurality of electrodes that cooperate to form a pattern of interdigitated electrodes. In another embodiment, the second film has a pair of conductive patterns similar to the conductive patterns of the first film.  
      In a related aspect, the present invention is directed to a piezoelectric macro-fiber composite actuator, comprising: 
          a structure consisting of piezo-electric material having a first side and a second side;     a first film bonded to the first side of the structure, the film further including first and second conductive patterns formed thereon, the first conductive pattern being electrically isolated from the second conductive pattern, both conductive patterns being in electrical contact with the piezo-electric material structure, the first and second conductive patterns each having a plurality of electrodes that cooperate to form a pattern of interdigitated electrodes; and     a second film bonded to the second side of the structure.        

      In a further aspect, the present invention is directed to a piezoelectric macro-fiber composite actuator, comprising: 
          a plurality of piezoelectric fibers in juxtaposition, each fiber having a first side and a second side, each pair of adjacent fibers being separated by a channel;     a first adhesive layer disposed over the first sides of the fibers and in the channel;     a first film bonded to the first sides of the fibers, the film further including first and second conductive patterns formed thereon, the first conductive pattern being electrically isolated from the second conductive pattern, both conductive patterns being in electrical contact with the piezo-electric material structure, the first and second conductive patterns each having a plurality of electrodes that cooperate to form a pattern of interdigitated electrodes;     a second adhesive layer disposed over the second sides of the fibers and into the channels; and     a second film bonded to the second sides of the fibers, the second film having a first conductive pattern and a second conductive pattern electrically isolated from the first conductive pattern of the second film, the first and second conductive patterns of the second film being in electrical contact with the fibers, the first and second conductive patterns of the second film each having a plurality of electrodes that cooperate to form a pattern of interdigitated electrodes.       

    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      The features of the invention are believed to be novel and the elements characteristic of the invention are set forth with particularity in the appended claims. The figures are for illustration purposes only and are not drawn to scale. The invention itself, however, both as to organization and method of operation, may best be understood by reference to the detailed description which follows taken in conjunction with the accompanying drawings in which:  
       FIG. 1  is a perspective view of a typical piezoelectric wafer.  
       FIGS. 2-7B  are perspective views illustrating preferred method steps of the present invention for making a piezoelectric macro-fiber composite actuator.  
       FIG. 8  is a top plan view of the assembled piezoelectric macro-fiber composite actuator having electrically conductive extensions attached thereto.  
       FIG. 9  is an exploded, perspective view illustrating an actuator fabricated in accordance with an alternate embodiment of the method of the present invention.  
       FIG. 10  is an exploded, perspective view illustrating an actuator fabricated in accordance with a further embodiment of the method of the present invention.  
       FIG. 11  is an exploded, perspective view illustrating an actuator fabricated in accordance with yet another embodiment of the method of the present invention.  
       FIGS. 12A and 12B  are perspective views illustrating an actuator fabricated in accordance with yet a further embodiment of the method of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
      In describing the preferred embodiments of the present invention, reference will be made herein to  FIGS. 1-12B  of the drawings in which like numerals refer to like features of the invention.  
     (1) Preferred Embodiment  
      Referring to  FIG. 1 , the first step of the method of the present invention entails providing a ferro-electric wafer  20 . For example, wafer  20  is fabricated from unelectroded, piezoelectric material. In one embodiment, PZT-5 piezoelectric ceramic material is used to fabricate the wafer  20 . However, it is to be understood that any piezo-electric material may be used to fabricate wafer  20 . In a preferred embodiment, piezoelectric wafer  20  has a thickness between about 0.002 and 0.010 inches.  
      Referring to  FIG. 2 , the next step entails disposing piezoelectric wafer  20  on a relatively thin polymer backing sheet  22 . In a preferred embodiment, the polymer backing sheet is moderately adhesive so as to facilitate handling during the subsequent steps of the fabrication method of the present invention.  
      Referring to  FIG. 3 , the next step comprises forming a plurality of slots or channels  24  on piezoelectric wafer  20 . While the slots  24  extend through substantially the entire thickness of wafer  20 , they do not completely slice the underlying polymer backing sheet  22 . This step results in the creation of a sheet of side-by-side piezoelectric macro-fibers  26  attached to the polymer backing layer  22 . In a preferred embodiment, slots  24  are formed by a machining process that uses a commercially available computer-controlled dicing saw. However, other cutting methods may be used, e.g. lasers. In a preferred embodiment, each slot  24  has substantially the same width, which is between about 0.001 and 0.005 inches. However, each slot  24  can have a width less than 0.001 inch or greater than 0.005 inch. In a preferred embodiment, each macro-fiber  26  has a width between about one (1) and (2) two times the thickness of piezoelectric wafer  20 . However, each macro-fiber  26  can have a width that is less than the thickness of piezoelectric wafer  20  or greater than twice the thickness of piezoelectric wafer  20 .  
      Referring to  FIG. 4 , the next step is to fabricate electrically a pair of non-conducting film elements that will be bonded to macro-fibers  26 . One such film element is film  28 . Film  28  can be fabricated from any type of electrically non-conducting material. In one embodiment, the electrically non-conducting material is fabricated from a polyimide. One suitable material is Kapton® manufactured and marketed by Dupont®. In a preferred embodiment, film  28  has a thickness between about 0.0005 and 0.001 inches. Preferably, film  28  has width and length dimensions which are larger than the width and length of piezoelectric wafer  20 . The reasons for this configuration will be discussed below.  
      Referring to  FIG. 4 , film  28  comprises two electrically conductive patterns  30  and  32 . Conductive pattern  30  comprises a longitudinally extending portion  34  and interdigiiated electrode fingers  36 . Conductive pattern  32  comprises a longitudinally extending portion  38  and interdigitated electrode fingers  40 . In one embodiment, conductive patterns or electrodes  30  and  32  are formed on film  28  using a photo-resist-and-etch process and pre-bonded polyimide-copper sheet laminate (e.g. Dupont® Pyralux® copper clad laminates). In a preferred embodiment, the thickness of the copper sheet material is between about 0.0005 and 0.001 inches. For example, a copper sheet having a thickness of about 0.0007 inch has provided good results. Although the foregoing description is in terms of conductive patterns  30  and  32  being fabricated from copper-sheet material, other types of sheet materials, e.g. gold, silver, etc, may also be used. The polyimide-conductive material laminate may also utilize an electro-deposited conductive layer instead of a pre-bonded conductive sheet, such as rolled and annealed copper.  
      Referring to  FIG. 4 , in a preferred embodiment, the center-to-center spacing of longitudinally extending portions  34  and  38  is about six times the thickness of piezoelectric wafer  20 , and the spacing between interdigitated electrodes or “fingers”  36  and  40  is about equal to the thickness of piezoelectric wafer  20 . The center-to-center spacing of longitudinally extending portions  34  and  38  and interdigitated electrodes or fingers  36  and  40 , however, can be other than described above. Furthermore, the width of conductive patterns  30  and  32  may have any suitable width.  
      Referring to  FIGS. 2-4 , film  28  has width and length dimensions that are larger than the width and length of piezoelectric wafer  20  so as to permit the placement of longitudinally extending portions  34  and  38  of conductive patterns  30  and  32 , respectively, away from piezoelectric wafer  20 . This configuration significantly lessens the potential for cracking of macro-fibers  26  caused by highly non-uniform electrical field distribution in regions beneath and adjacent to the longitudinally extending portions  34  and  38 . Additionally, this packaging concept affords a sealed electrical system protected from the environment.  
      Referring to  FIGS. 4 and 5 , a second film  42  is fabricated in accordance with the steps described above. In one embodiment, film  42  comprises conductive patterns or electrodes  44  and  46 . Conductive pattern  44  comprises longitudinally extending portion  48  and interdigitated electrodes or fingers  50 . Similarly, conductive pattern  46  comprises longitudinally extending portion  52  and interdigitated electrodes or fingers  54 . Conductive patterns  44  and  46  of film  42  are “mirror images” of conductive patterns  30  and  32 , respectively, of film  28 . The next step comprises positioning films  28  and  42  as shown in  FIG. 5  such that film  28  confronts one side or face of macro-fibers  26  and film  42  confronts the other side of macro-fibers  26 . Conductive patterns  30  and  32  of film  28  are directly aligned with conductive patterns  44  and  46  of film  42 . Thus, conductive patterns  30  and  32  are in “mirror-image” alignment with conductive patterns  44  and  46  across the thickness of macro-fibers  26 . Although film  42  has been described in the foregoing description as having conductive patterns thereon, film  42  may be configured without any conductive patterns.  
       0019 ]Referring to  FIGS. 6A, 6B ,  7 A, and  7 B, films  28  and  42  are bonded with an adhesive to macro-fibers  26  to form a flexible laminate. In a preferred embodiment, the adhesive is a two-part liquid epoxy to bond films  28  and  42  to macro-fibers  26 . An example of such a liquid epoxy is Scotchweld DP-460 epoxy manufactured by 3M Company. However, other types of bonding materials can be used, e.g. urethane, acrylic, etc. Referring to  FIG. 6A , the first step in the bonding process is to coat the electrode face of film  42  with a relatively thin layer of liquid epoxy. Referring to  FIG. 6B , sheet  22  and macro-fibers  26  are then placed on film  42  such that macro-fiber  26  contacts the epoxy-coated face of electrode film  42 . Light pressure, indicated by arrow  56 , and heat are applied in a vacuum to partially cure the epoxy layer to affix the macro-fibers to electrode film  42 . After the partial cure is complete, polymer backing sheet  22 , previously used for handling of macro-fibers  26 , is peeled away and discarded. Referring to  FIG. 7A , macro-fibers  26  are now attached to the bottom electrode film  42  by the epoxy. An additional coat of liquid epoxy is now applied to macro-fibers  26  in order to fill all machined slots  24  between adjacent fibers  26 . Application of epoxy in this manner serves to substantially eliminate air pockets between adjacent, alternately charged electrode fingers  36 ,  40 ,  50  and  54  in the final assembly. The elimination of these air pockets substantially reduces the probability of electrical arcing or permanent shorts which would render the actuator inoperable.  
      Referring to  FIGS. 6B, 7A , and  7 B, after slots  24  are filled with the epoxy, the next step is to apply a relatively thin coat of epoxy to the electroded face of upper film  28 . Next, film  28  is placed epoxy side down onto the previously coated surface of macro-fibers  26  such that conductive electrode patterns  30 ,  32  and  44 ,  46  of films  28  and  42 , respectively, are substantially aligned. The next step entails applying moderate pressure, indicated by arrow  58 , and heat to the assembly of films  28 ,  42  and macro-fibers  26 . The heat and pressure are applied in a vacuum until a substantially complete, void-free cure of the epoxy is attained. Application of this pressure also forces the relatively thick copper conductive patterns or electrodes  30 ,  32  and  44 ,  46  to contact and rest upon the flat surfaces of the macro-fibers  26 . Such contact between the relatively thick copper conductive patterns or electrodes  30 ,  32  and  44 ,  46  and the flat surfaces of macro-fibers  26  creates a bond line between the conductive patterns or electrodes  30 ,  32  and  44 ,  46  and fiber  26  that is extremely thin or “starved,” resulting in only a minimal attenuation of the actuator&#39;s electric field produced when voltage is applied. The bond line between the unelectroded portions of films  28  and  42  (i.e. the portions of films  28  and  42  having no conductive pattern) and fibers  26  is sufficiently thick to keep films  28  and  42  attached. This process results in a longitudinal mode piezoelectric fiber actuator  10 .  
      As shown in  FIG. 8 , conductive patterns  30  and  32  are provided with electrically conductive extensions  68  and  70 , respectively. During operation, an external power supply (not shown) is electrically connected to the extensions  68  and  70  in a manner such that at any one moment in time, opposite electrical polarity is supplied to interdigitated fingers  36 ,  40  and  50 ,  54 . This polarity generates electric fields directed along the length of fibers  26  in the regions between adjacent interdigitated electrode fingers  36  and  40  and between fingers  50  and  54 .  
      The interdigitated electrodes  36 ,  40  and  50 ,  54  are also used for polarizing the piezoelectric fibers  26 . Polarization of the macro-fibers  26  is typically required before operating the device as an actuator. Polarization is performed by applying a steady voltage across alternate electrode fingers  36 ,  40  and  50 ,  54 . In one embodiment, a voltage which generates an average electric field intensity of approximately 300% of the room temperature coercive electric field of the macro-fibers  26  is used. Such voltage is applied to the actuator for approximately 20 minutes at room temperature. Other poling techniques, as are well understood in the art, may also be used.  
      Subsequent application of a voltage to conductive patterns  30 ,  32 ,  44 , and  46  produces an induced strain in macro-fibers  26 . The largest strain produced occurs along the fiber length direction, with a contractile strain occurring in the transverse direction.  
     (2) Alternate Embodiments  
       FIG. 9  depicts an alternate piezoelectric fiber actuator  100  of the present invention. Shear-mode actuator  100  is configured to allow continuous twisting moments to be easily produced in a host structure, e.g. high aspect ratio structures, beams, spars, etc. Shear-mode actuator  100  generally comprises films  102 ,  104  and piezoelectric fibers  106 . Films  102 ,  104  and fibers  106  are adhesively bonded together using an epoxy as described above. Piezoelectric fibers  106  have separated slots  108  which are the result of a cutting or slicing process as has been previously described. Fibers  106  define a longitudinally extending edge  110 . Slots  108  are formed at an angle with respect to longitudinally extending edge  110 . Preferably, each slot  108  is formed at a 45° angle with respect to the longitudinal extending edge  110  because such an angular orientation provides optimum results in inducing piezoelectric shear stresses within a host structure. However, slots  108  may be formed at a different set of angles with respect to the longitudinally extending edge  110 .  
      Film  102  includes two conductive patterns  112  and  114  formed thereon. Conductive pattern  112  includes a longitudinally extending portion  116  and interdigitated electrodes or fingers  118 . Similarly, conductive pattern  114  includes a longitudinally extending portion  120  and interdigitated electrodes or fingers  122 . As shown in  FIG. 9 , fingers  118  are angulated with respect to longitudinally extending portion  116 . Similarly, fingers  122  are angulated with respect to longitudinally extending portion  120 . In a preferred embodiment, fingers  118  and  122  are formed at a 45° angle with respect to portions  116  and  120 , respectively, so that fingers  118  and  120  are substantially perpendicular to the fibers  106 .  
      In one embodiment, film  104  includes two conductive patterns  124  and  126  formed thereon. Conductive pattern  124  includes a longitudinally extending portion  128  and interdigitated electrodes or fingers  130 . Similarly, conductive pattern  126  includes a longitudinally extending portion  132  and interdigitated electrodes or fingers  134 . As shown in  FIG. 9 , fingers  130  are angulated with respect to longitudinally extending portion  128 . Similarly, fingers  134  are angulated with respect to longitudinally extending portion  132 . In a preferred embodiment, fingers  130  and  134  are formed at a  450  angle with respect to portions  128  and  132 , respectively, so that fingers  130  and  134  are substantially perpendicular to the fibers  106 . Although film  104  has been described in the foregoing description as having conductive patterns thereon, film  104  may also be configured without any conductive patterns. Films  102  and  104  are bonded with an adhesive to macro-fibers  106  in a process similar to the process previously described for assembly of piezoelectric fiber actuator  10  and shown by  FIGS. 6A, 6B ,  7 A, and  7 B.  
      Actuator  100  further includes four electrical conductors (not shown) wherein each electrical conductor is electrically connected to a corresponding one of conductive patterns  112 ,  114 ,  124 , and  126 . In a preferred embodiment, each of the electrical conductors are positioned near the edge of films  102 ,  104  and function to electrically connect actuator  100  to external electronic circuitry (not shown). The four electrical conductors apply electrical power to actuator  100  in the same manner as described above.  
       FIG. 10  illustrates a further embodiment of the actuator of the present invention. Actuator  200  generally comprises a plurality of piezoelectric macro-fibers  202  separated by slots  204 , and films  206 ,  208 ,  210 , and  212 . Slots  204  are formed by the slicing or cutting methods previously described herein. Films  206  and  208  are generally the same in construction as films  28  and  42 , respectively, discussed above.  
      Film  206  includes two conductive patterns  214  and  216  formed thereon. Conductive pattern  214  includes a longitudinally extending portion  218  and interdigitated electrodes or fingers  220 . Similarly, conductive pattern  216  includes a longitudinally extending portion  222  and interdigitated electrodes or fingers  224 . As shown in  FIG. 10 , fingers  220  and  224  are substantially perpendicular to longitudinally extending portions  218  and  222 , respectively.  
      In one embodiment, film  208  comprises two conductive patterns  226  and  228 . Conductive pattern  226  includes a longitudinally extending portion  230  and interdigitated electrodes or fingers (not shown). Similarly, conductive pattern  228  includes a longitudinally extending portion  232  and interdigitated electrodes or fingers  236 . The fingers of film  208  are substantially perpendicular to longitudinally extending portions  230  and  232 . Film  208  may also be configured without any conductive patterns.  
      Actuator  200  further comprises anisotropically conductive films or sheets  210  and  212  positioned on the top and bottom of piezoelectric macro-fibers  202 . Each film  210  and  212  has generally the same surface area as the total surface area of piezoelectric macro-fibers  202 . Films  210  and  212  are used to bond films  206  and  208  to the piezoelectric macro-fibers  202 . Each film  210  and  212  comprises a thermoset/thermoplastic adhesive matrix. In one embodiment, the adhesive matrix has a thickness between about 0.0001 and 0.002 inches. The adhesive matrix has randomly loaded conductive particles. These conductive particles provide conductive paths through the thickness of the adhesive film, but not through the plane of the film. This pathing arrangement permits the fingers of films  206  and  208  to be in direct electrical contact with the underlying piezoelectric fibers  202  while remaining electrically isolated from adjacent, oppositely charged fingers. In one embodiment, the conductive particles have a diameter of about 0.0005 inch. Films  210  and  212  comprise Z-Axis Film, product no. 3M 5303R, manufactured by 3M Company, Inc. However, other films having generally the same anisotropically conductive characteristics as the aforementioned Z-Axis Film may be used.  
      Referring to  FIG. 10 , before final assembly of actuator  200 , slots  204  are filled with an electrically non-conductive matrix epoxy to prevent the development of air pockets. The application of the epoxy is implemented in generally the same manner as previously described for assembly of actuator  10 .  
      Referring to  FIG. 10 , the use of films  210 ,  212  to bond films  206  and  208  to piezoelectric macro-fibers  202  creates relatively strong bond lines that are maintained beneath and between fingers of films  206  and  208 . In an alternate embodiment, films  206  and  208  may be added during the fabrication of the shear-mode actuator previously described and shown in  FIG. 9 .  
       FIG. 11  shows another embodiment of the actuator of the present invention. Actuator  300  generally comprises a monolithic piezoelectric wafer  302  and films  304  and  306 . Wafer  302  may be produced as a longitudinal-mode or shear-mode actuator. Films  304  and  306  have electrode patterns and are generally the same in construction as films  28  and  42  described above and shown in  FIGS. 4 and 5 .  
      Film  304  comprises a conductive pattern  308  which has a longitudinally extending portion  310  and interdigitated electrodes or fingers  312 . Film  304  further comprises conductive pattern  314 , which has a longitudinally extending portion  316  and interdigitated electrodes or fingers  318 . As shown in  FIG. 11 , fingers  312  and  318  are substantially perpendicular to longitudinally extending portions  310  and  316 , respectively.  
      In one embodiment, film  306  comprises a conductive pattern  320  having a longitudinally extending portion  322  and interdigitated electrodes or fingers  324 . Film  306  further comprises a conductive pattern  326  having a longitudinally extending portion  328  and interdigitated electrodes or fingers  330 . As shown in  FIG. 11 , fingers  324  and  330  are substantially perpendicular to the longitudinally extending portions  322  and  328 , respectively. Film  306  may also be configured without any conductive patterns.  
      Films  304  and  306  may be bonded to wafer  302  by any of the methods previously described. The omission of the machined slots in wafer  302  significantly reduces the per-unit cost of actuator  300  and provides a relatively high actuation-efficiency device. Additionally, the lamination effect of the attached electrode films  304  and  306  provides actuator  300  with a predetermined degree of flexibility and conformability which, although not as great as actuators  10 ,  100  and  200 , makes actuator  300  suitable for applications wherein endurance and fatigue life are not major considerations, for example, launch vehicle payload shrouds, torpedo bodies, missile stabilizer fins, etc.  
      A further embodiment of the actuator of the present invention is given in  FIGS. 12A and 12B . The first step in fabricating actuator  400  is to bond together a plurality of relatively thin piezoelectric wafers  402  to form a stack  404 . In a preferred embodiment, a liquid epoxy as previously described is used to bond together the wafers  402 . Stack  404  may be of almost any height. In one embodiment, the height of stack  404  is about 0.25 inch. In a preferred embodiment, the thickness of bond lines  406  between adjacent wafers  402  is between about 0.125 and 0.25 times the nominal thickness of the individual piezoelectric wafers  402 . After stack  404  is bonded, it is cured at relatively moderate pressure and temperature to form a substantially void-free bonded stack. In a preferred embodiment, the aforementioned pressure and temperature are applied under a vacuum.  
      Next, stack  404  is sliced parallel to the thickness direction and along the length direction, as indicated by dotted lines  408 , to provide a plurality of relatively thin, piezoelectric sheets  410 . In one embodiment, a wafer dicing saw is used to cut fiber sheets  410 . However, other cutting methods may be used. Fiber sheets  410  may be handled and packaged in the same manner as monolithic piezoelectric wafers. In one embodiment, the thickness of each sheet  410  is about equal to the thickness of one of the piezoelectric wafers  402  used to form stack  404 . However, each sheet  410  may have a thickness that is less than or greater than the thickness of one of the piezoelectric wafers  402 .  
      Referring to  FIG. 12B , sheet  410  is positioned between films  412  and  414 . Film  412  comprises a conductive pattern  416 , which has a longitudinally extending portion  418  and interdigitated electrodes or fingers  420 , and a conductive pattern  422 , which has a longitudinally extending portion  424  and interdigitated electrodes or fingers  426 . As shown in  FIG. 12B , fingers  420  and  426  are substantially perpendicular to longitudinally extending portions  418  and  424 , respectively.  
      Film  414  comprises a conductive pattern  428  having a longitudinally extending portion  430  and interdigitated electrodes or fingers  432 . Film  414  further comprises a conductive pattern  434  having a longitudinally extending portion  436  and interdigitated electrodes or fingers  438 . Fingers  432  and  438  are substantially perpendicular to longitudinally extending portions  430  and  436 , respectively. Film  414  may also be configured without any conductive patterns. Films  412  and  414  are adhesively bonded to sheet  410  via a liquid epoxy or using an anisotropically conductive film as previously described.  
      The configuration shown in  FIGS. 12A and 12B  has two significant advantages. First, the possibility of bonding to a surface skin is virtually eliminated. Second, all the macro-fibers of sheets  410  are pre-aligned.  
     (3) Advantages Over Prior Art Actuators And Methods  
      The method of the present invention substantially eliminates the need to manufacture and individually handle large numbers of piezoelectric fibers. Thus, production time and handling costs associated with packaging piezoelectric fiber composite actuators are significantly reduced. The method of the present invention is easily controlled and precise, which greatly enhances the repeatability and uniformity of the actuators produced. The method of the present invention permits square fibers to be manufactured and easily aligned within the actuator package without the possibility of damage to the actuator electrodes. Thus, the difficulties associated with the use of square cross-section piezoelectric fibers are virtually eliminated. The use of square fibers in accordance with the present invention instead of round fibers allows the volume fraction of piezoelectric material within the actuator package to be increased, thereby improving the actuation stress capability of the actuator. The use of the relatively thick copper conductive patterns, which are attached via liquid epoxy or anisotropically conductive adhesive, also provide for an unimpeded electrical connection to be made between the piezoelectric material and the electrodes. As a result, the electric field transfer efficiency of the actuator electrodes is significantly improved, which in turn increases the strain produced per unit applied voltage. A further advantage is that the square or rectangular fibers have a substantially flat contact area with the electrodes. This flat contact area is relatively greater than the contact area achieved with round fibers.  
      The polyimide films each have width and length dimensions that are larger than the width and length of piezoelectric wafer so as to permit the placement of longitudinally extending portions of the conductive patterns (e.g. portions  34  and  38  of conductive patterns  30  and  32 , respectively) away from the piezoelectric wafer. This configuration significantly lessens the potential for cracking of the macro-fibers caused by highly non-uniform electrical field distribution in regions beneath and adjacent to the longitudinally extending portions of the conductive patterns. Additionally, this packaging concept affords a sealed electrical system that is protected from the environment.  
      While the present invention has been particularly described, in conjunction with a specific preferred embodiment, it is evident that many alternatives, modifications, and variations will be apparent to those skilled in the art in light of the foregoing description. It is therefore contemplated that the appended claims will embrace any such alternatives, modifications, and variations as falling within the true scope and spirit of the present invention.