Patent Document

RELATED APPLICATIONS 
       [0001]    The present application claims priority to U.S. Provisional Patent Application Ser. No. 61/236,003, entitled PIEZOELECTRIC ELECTROSTRICTIVE COMPOSITION DEVICE AND METHOD, filed on Aug. 21, 2009, the disclosure of which is hereby incorporated by reference. 
     
    
     BACKGROUND 
       [0002]    Piezoelectric and electrostrictive materials are used in industrial and consumer applications for in the case of the former creating an electric field in the presence of mechanical tension or, in the case of both, conversely for changing their geometry in the presence of an electric field. Actuators are devices made from such materials that are used to provide, e.g., displacement of loads in the tenths of Newtons (N), with sub-mm accuracies in displacement. 
         [0003]    The piezoelectric effect is understood as the linear electromechanical interaction between the mechanical and the electrical state, e.g., in crystalline materials with no inversion symmetry. The piezoelectric effect is a reversible process in that materials exhibiting a direct piezoelectric effect (the internal generation of electrical charge resulting from an applied mechanical force) also exhibit the reverse piezoelectric effect (the internal generation of a mechanical force resulting from an applied electrical field). For example, lead zirconate titanate (“PZT”) crystals will generate measurable piezoelectricity when the static structure of the material is deformed to about 0.1% of the original dimension. Conversely, PZT crystals will change about 0.1% of their static dimension when an external electric field is applied to the material. 
         [0004]    Electrostriction is a property of all dielectric materials, and is caused by the presence of randomly-aligned electrical domains within the material. When an electric field is applied to the dielectric, the opposite sides of the domains become differently charged and attract each other, thereby reducing material thickness in the direction of the applied field (and increasing thickness in the orthogonal directions due to Poisson&#39;s ratio). The resulting strain (ratio of deformation to the original dimension) is proportional to the square of the polarization. Reversal of the electric field, however, does not reverse the direction of the deformation. 
         [0005]    More formally, the electrostriction coefficient is a fourth rank tensor (Qu ijkl ), relating second order strain (x ij ) and first order polarization tensors (P k , P l ), such that: 
         [0000]    
       
      
       I 
       ij 
       =Q 
       ijkl 
       ×P 
       k 
       ×P 
       l  
      
     
         [0006]    The related piezoelectric effect occurs only in a particular class of dielectrics. Electrostriction applies to all crystal symmetries, while the piezoelectric effect only applies to the 20 piezoelectric point groups. Electrostriction is a quadratic effect, unlike piezoelectricity, which is a linear effect. In addition, as noted above, and unlike piezoelectricity, electrostriction cannot be reversed, i.e., deformation will not induce an electric field. 
       SUMMARY 
       [0007]    According to aspects of an embodiment of the disclosed subject matter, a “Bi-Layer Piezoelectric/Electrostrictive (P/E) Dome Unimorph” [BIPEDU] is disclosed. Such an actuator may be composed of piezoelectric layer(s), such as, a lead magnesium niobate-lead titanate (“PMN/PT”)-65/35 and an electrostrictive layer(s), such as PMN/PT-90/10. Such an actuator may be manufactured by co-pressing and co-sintering these layers together as a monolithic construction. Internal stress can be generated after fabrication due, e.g., to thermal expansion mismatch of the two materials which can result in a domed shape and enhanced actuation performance. The dome may be attached to a rigid substrate, such as a rigid plate that can be used to prevent the movement of the perimeter of the base of the dome, which can result in a dramatic improvement of the performance of the actuator. 
         [0008]    Such an actuator may be used as a component in electronic devices requiring a large and precise displacement, moderate load bearing and low electrical loss such as micropositioners, noise and vibration canceling devices, micropumps, deformable mirrors, loudspeakers, laser and other light deflectors, damping devices, auto focus cameras and, in high reliability applications requiring reasonable actuation performance like precision machining, optics, astronomy and fluid control. 
         [0009]    According to aspects of an embodiment of the disclosed subject matter, an actuator is provided that can have a significant amplification of displacement, a narrower displacement hysteresis loop and reduced electromechanical loss, e.g., due to the monolithic combination of materials with piezoelectric and the electrostrictive effects. According to other aspects of embodiments of the disclosed subject matter a faster, simpler and cheaper manufacturing process is an advantage of the construction of the disclosed actuator. An actuator is provided which may have a plurality of piezoelectric material layers, a plurality of electrostrictive material layers, and means to prevent the movement of the perimeter, such as, the circumference of a dome constructed of the materials according to aspects of an embodiment of the disclosed subject matter. In such an actuator a plurality of piezoelectric layers can consist of one layer and the plurality of electrostrictive layers consist of one layer. Such an actuator may also have a plurality of piezoelectric material layers, and means to prevent the movement of the perimeter or the circumference, e.g., of a dome forming the actuator. Alternatively there may be a plurality of electrostrictive layers, and means to prevent the movement of the perimeter or the circumference of the dome. Such an actuator may be formed by co sintering and co pressing a plurality of layers as a monolithic construction. The actuator may be made with PMN/PT-65/35 piezoelectric material and PMN/PT-90/10 electrostrictive material. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]      FIG. 1  shows a schematic of a Bi-Layer Piezoelectric/Electrostrictive (P/E) Dome Unimorph (BIPEDU) according to aspects of an embodiment of the disclosed subject matter; 
           [0011]      FIG. 2   a  illustrates graphically displacement under an electric field of 10 kV/cm of a dome actuator and BIPEDU with a P/E 6/4 ratio; 
           [0012]      FIG. 2   b  illustrates graphically displacement under an electric field of 10 kV/cm of dome actuator and a BIPEDU with a P/E 7/3 ratio; 
           [0013]      FIG. 2   c  illustrates graphically displacement under an electric field of 10 kV/cm of a dome actuator and a BIPEDU with a P/E 8/2 ratio; 
           [0014]      FIG. 3   a  illustrates graphically displacement versus load of a BIPEDU dome actuator at P/E 6/4, 7/3 and 8/2 volume ratios under an electric field 10 kV/cm; 
           [0015]      FIG. 3   b  illustrates graphically displacement versus load of a BIPEDU at P/E 6/4, 7/3 and 8/2 volume ratios under an electric field 10 kV/cm; 
           [0016]      FIG. 4  illustrates graphically cyclic displacement to load behavior of BIPEDU at P/E 6/4 volume ratio; and, 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0017]    Applicants have determined that an improved performance bi-layer actuator  10 , such as is illustrated schematically in  FIG. 1 , can be fabricated from materials with different thermal expansion coefficients (TEC) EC) which create internal stress after manufacturing. This improves the mechanical performance and can shape the device  10  as a dome. The manufacturing process for use of piezoelectric and electrostrictive materials is generally known as sintering. A “green body” can be formed by pressing the powder that is generally produced by spray drying a slurry comprising the piezoelectric or electrostrictive material, binders and liquid. The body can then heated to a low temperature, to burn off the binder, followed by sintering at a high temperature beyond the melting point to fuse particles together. Sintering can be done with pressure (hot pressing) or without. 
         [0018]    Turning now to  FIG. 1 , there is shown a BIPEDU  10 , i.e., a bi-layer P/E unimorph dome composed of piezoelectric material  12 , such as PMN/PT-65/35 and electrostrictive material  14 , such as, PMN/PT-90/10 layers as the electrically active elements. Internal stress within the element  10  can be generated during fabrication due to the different thermal expansion coefficient (TEC) of the two materials  12 ,  14 . The dome bi-Layer actuator  10  has its perimeter firmly attached to a rigid plate  20  which can improve dramatically the actuation performances under an external E-field. During the above mentioned sintering process in a boundary between the piezoelectric layer  12  and the electrostrictive layer  14  the piezoelectric layer  12  and electrostrictive layer  14  can be caused to fuse into a monolithic structure that assists in bonding the piezoelectric layer  12  and electrostrictive layer  14  together. 
         [0019]    In a preferred embodiment epoxy resin  30  can be applied around the circumference of the actuator  10  as shown schematically in  FIG. 1 . That can result in a dramatic improvement of the actuation performance. 
         [0020]    The device  10  can be manufactured using, e.g., a conventional dry pressing method using fewer steps for the bi-layer green ceramic preparation. This process can be faster and cost less. Piezoelectric and electrostrictive powders can be individually dry-pressed on top of each other at pressure such as 86, 114 and 229 MPa into pellets with 14.7 mm diameter and 1.15-1.3 mm thickness using a uni-axial hydraulic machine. Once the dimensions and masses of green pellets are recorded, the binder removal and pre-sintering can be performed at 550° C. for 3 hours and 780° C. for 1 hour, respectively. Then the same parameters can be recorded before and after the sintering step. The pellets can then be sintered in a closed alumina crucible with 3 grams of PbZrO 3  source, at 1150, 1200 and 1250° C. for 1 hour. The heating and cooling rate can be set at 3° C./min during sintering. A monolithic layer containing both the piezoelectric material and the electrostrictive material forms at the boundary of the two and enhances adhesion between the respective layers. 
         [0021]    The BIPEDU  10  can be used as an actuator through the converse piezoelectric effect where an electric field is applied across the actuator  10  to generate strain and displacement in the micron-scale range. When an electric field is applied in the same direction as poling, both the piezoelectric and electrostrictive layers contract and the center of the element  10  moves up, as illustrated schematically in  FIG. 1 . The metal plate  20  attached to the bi-layer dome actuator  10  can act as a support which provides more force to push up the bi-layer P/E element  10  and can thus generate additional displacement and strain amplification. The monolithic combination of materials of the piezoelectric layer  12  and electrostrictive layer  14 , within the boundary layer between the piezoelectric layer  12  and electrostrictive layer  14 , can reduce electromechanical loss and narrow displacement of the hysteresis loop. By using the two active piezoelectric and electrostrictive materials, the sum of the transverse piezoelectric coefficient (d 33 ) effective can be converted to additional displacement in the longitudinal direction (toward the top of the page in the illustration of  FIG. 1 ) under an applied electric field. 
         [0022]    The enhanced actuation performance of the BIPEDU  10  makes it suitable for many applications including but not limited to nanopositioning, active and adaptive optics, vibration cancellation, pneumatic valves, micropumps and laser tuning which requires high resolution and high precision displacement. 
         [0023]    Aspects of embodiments of the disclosed subject matter may be described more fully by way of the following non-limiting examples. 
       Example 1 
       [0024]    In an embodiment two different compositions of lead magnesium niobate-lead titanate (PMN/PT) were used to form the bi-layer dome actuators  10 . Piezoelectric PMN/PT-65/35 and electrostrictive PMN/PT-90/10 powders were mixed with a 20 wt % polyvinyl alcohol (PVA) solution at a 6 wt % concentration separately, and then heated in an oven for 15 minutes at 120° C., ground and sieved to obtain uniformly fine powder. Bilayer actuator  10  structures with various volume ratios of piezoelectric and electrostrictive powders were prepared by uniaxial co-pressing at 86 MPa pressure in a die with the diameter of 15 mm. The thickness of the pressed bi-layer sample was about 1.2 mm. After heating for binder removal at 550° C. for 3 hours, the bi-layers were co-sintered at 1200° C. with the dwell time of 1 hour. An excess Pb source was placed in the crucible to suppress Pb losses during sintering. The samples were curved slightly after sintering with dome height in the range of 0.3-0.45 mm. The concave side (piezoelectric layer  12 ) of the bi-layer P/E actuator  10  was then slightly polished in the edge area with SiC sandpaper (1200 grit) to create a contact ring area where the metal plate was attached to the bi-layer P/E dome actuator, e.g., with epoxy resin. 
       Example 2 
       [0025]    In an embodiment of the actuator device  10 , rigid metal plates  20  (76 pm thick stainless steel) with essentially identical diameter to the bi-layer P/E actuators  10  were prepared using a punching tool. To attach the bi-layer structure  10  and metal plate  20 , the epoxy resin  30  (available from Emerson and Cuming Inc., Billerica, Mass., under the product designation of Eccobond 15 LV and Catalyst 15 LV) was applied around the polished circumferential area, aligning the whole assembly  10  in a fixture used to attach the element  10  to the plate  20 . In this example, the Eccobond/Catalyst mixing ratio of 3/1 (by weight) was used to obtain a bonding layer with moderate stiffness. The thickness of the cured epoxy was about 20 μm. Then the fixture was placed into the oven at 65° C. for 3 hours to harden the epoxy resin. The device  10  can be embodied in a miniature actuator  10  (such as, having a 10 mm diameter and 1.2 mm thickness) with a moderate displacement (up to 70 microns @ 1000 V) and moderate load (up to 10 N @ 1000 V) that is well suited for nanopositioning/high speed switching, miniature pneumatic and hydraulic valves, micropumps, laser tuning and active and adaptive optic and vibration cancellation applications. 
       Example 3 
       [0026]    The device  10  described in example 2 was further tested by electroding it with fire-on silver paste and heat treated at 550° C. for 15 minutes. Poling was carried out with a 25 kV/cm electric field strength in a heated silicone oil bath for 10 minutes at 150° C., followed by a cool-down to room temperature under the same poling field strength. The electromechanical properties of the bi-layer P/E monolithic composites  10  were characterized after 1 day of aging at room temperature. The effective piezoelectric coefficient d 33  effective of the poled actuator  10  was measured directly with a Berlincourt d 33 -meter piezometer at 100 Hz. The room temperature dielectric constant and loss tangent of the samples were measured at 1 kHz using an HP 4194A impedance/gain-phase analyzer (available from Hewlett Packard, Palo Alto, Calif., under the model number HP 4194A). The field-induced displacement was also investigated by a photonic sensor (available from MTI instruments, Latham, N.Y., under the model number MTI 2000). 
         [0027]    The electric field induced displacements of actuators  10  were determined using a DC unipolar wave form, which was coupled to a high voltage displacement fixture (HVDF) provided by Radiant Technologies Inc. of New Mexico, U.S.A., under the name High Voltage Interface). A photonic fiber optic sensor probe (photonic sensor, model number MTI 2000) was used to measure the displacement of the actuator  10  under an applied field. Specifically, the BIPEDU actuator  10  was placed between two copper electrodes of the HVDF and loaded with 0.5 N forces which were exerted by a top copper electrode. Also the load-displacement behavior of the actuator  10  was investigated by placing the external load on top of the upper copper electrode. 
         [0028]    The electromechanical properties (d 33  effective, relative permittivity (K), tan  8  and displacement) of bilayer P/E component(s)  10  (before being attached to a metal plate) with P/E 6/4, 7/3 and 8/2 ratios were recorded. The displacement of a sample BIPEDU  10  is shown in Table 1. 
         [0000]    
       
         
               
               
               
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                   
                 d 33  eff 
                   
                   
                 Displacement (μm) 
                 Displacement (μm) 
               
               
                 Designation 
                 (pC/N) 
                 K 
                 % tan 6 
                 Dome P/E component 
                 BIPEDIJ 
               
               
                   
               
             
             
               
                 P/E: 6/4 
                 372 ± 22 
                 4,370 ± 230 
                 −2.26 ± 0.04  
                 10.38 ± 0.48  
                 73 ± 1.98 
               
               
                 P/E: 7/3 
                 520 ± 26 
                 3,715 ± 70  
                 1.93 ± 0.14 
                   8 ± 0.40 
                 69 ± 4.75 
               
               
                 P/E: 8/2 
                 550 ± 10 
                 3,370 ± 100 
                 1.72 ± 0.07 
                 5.48 ± 0.28 
                 65 ± 3.89 
               
               
                   
               
             
          
         
       
     
         [0029]    The comparison of the actuation performance between bi-layer P/E Dome acutaors which were not attached to a rigid metal plate, as noted above, and BIPEDU actuators  10  at P/E 6/4, 7/3 and 8/2 volume ratios are depicted in  FIGS. 2   a - 2   c  respectively. A dc electric field 10 kV/cm was applied to the comparison dome actuators and BIPEDU actuators  10  and the displacement signals at the center of the dome were detected by the above mentioned photonic sensor. The measured BIPEDU displacement of 65-73 μm was 7-12 times higher than that of the comparison bi-layer P/E dome actuator (5-11 μm). The displacements vs. load results are shown in  FIGS. 3   a - b  for both comparison bi-layer P/E dome actuators ( FIG. 3   a ) and BIPEDU actuators  10  ( FIG. 3   b ). The dome actuators  10  showed that they can tolerate load over 10 N. The excellent performance of the dome actuator components under the applied load is due to the dome geometry and also due to free movement of the actuator in the circular base plane (no adhesive was applied in this stage). On the other hand, the BIPEDU actuator  10  displacement decreased as the applied load was increased and showed the displacement in the range of 3-6 microns at 11 N. This is because the bi-layer actuators were glued to metal plate with resulting restraint in the movement of the overall actuator  10  in the plane under applied load. 
         [0030]      FIG. 4  depicts cyclic displacement-load behavior of representative BIPEDU&#39;s  10  with a P/E 6/4 ratio. In this experiment, three cycles of load-displacement data were consecutively recorded from 0.5-11 N with 1 N intervals and an E-field applied at 10 kV/cm. The results showed a precise load-displacement trend in each cycle. 
         [0031]    It is understood that one skilled in the art would be able to comprehend additional embodiments based on the teaching herein, which embodiments are intended to be included in this application.

Technology Category: h