Patent Publication Number: US-6703765-B2

Title: Devices using a ceramic piezoelectric

Description:
This is a continuation of application Ser. No. 09/798,707 filed on Mar. 2, 2001, now U.S. Pat. No. 6,517,737. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to piezoelectric materials and devices that use piezoelectric materials. 
     2. Discussion of the Related Art 
     Piezoelectric materials respond to applied electric fields by physically deforming. The magnitude of the deformations that electric fields generate in single-crystal perovskites such as Pb(Mg 1/3 Nb 2/3 )O 3 —PbTiO 3  (PMN-PT) and Pb(Zn 1/3 Nb 2/3 )O 3 —PbTiO 3  (PZN-PT) are an order of magnitude larger than those that the fields generate in polycrystalline piezoelectrics such as PbZrO 3 —PbTiO 3  (PZT). The size of their piezoelectric responses make the crystalline perovskites promising materials for new piezoelectric devices. 
     Unfortunately, the fabrication of crystalline perovskites is complicated and expensive. On the one hand, high fabrication costs make the crystalline perovskites too expensive for use in many types of electromechanical devices. On the other hand, polycrystalline PZN-PT typically require high-pressure synthesis, which is prohibitively expensive to some commercial applications of PZN-PT. 
     Herein, PMN refers to Pb(Mg 1/3 Nb 2/3 )O 3  and PT refers to PbTiO 3 . 
     Herein, chemical symbols are used for lead (Pb), niobium (Nb), zinc (Zn), zirconium (Zr), magnesium (Mg), titanium (Ti), and oxygen (O). 
     SUMMARY OF THE INVENTION 
     In one aspect, the invention features an electromechanical device. The electromechanical device includes a support structure, a component moveable with respect to the support structure, and a piezoelectric device mechanically coupled to both the support structure and the component. The piezoelectric device includes a polycrystalline body and electrodes located on the body. The body has a composition with a stoichiometry described by [Pb(Mg 1/3 Nb 2/3 )O 3 ] (1−x)  [PbTiO 3 ] x . The value of x is in the range of about 0.31 to about 0.47. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows x-ray diffraction patterns of ceramic compositions with the stoichiometry (PMN) (1−x)  (PT) x ; 
     FIG. 2 shows the piezoelectric responses of the compositions of FIG. 1; 
     FIG. 3 is a cross-sectional view of a slab-shaped piezoelectric device; 
     FIG. 4 is a flow chart showing a process for fabricating the piezoelectric device of FIG. 3; 
     FIG. 5 is an oblique view of an inkjet print head that uses the piezoelectric device of FIG. 3; 
     FIG. 6 is an oblique view of a micro-electro-mechanical (MEM) device that uses the piezoelectric device of FIG. 3; and 
     FIG. 7 is a cross-sectional view of a pressure gage that uses the piezoelectric device of FIG.  3 . 
    
    
     DETAILED DESCRIPTION 
     Various embodiments include polycrystalline compositions of Pb(Mg 1/3 Nb 2/3 )O 3  and PbTiO 3 . These polycrystalline compositions are stoichiometrically described by PMN (1−x)  PT x . The compositions have piezoelectric responses to applied electric fields. The magnitudes of the piezoelectric responses depend on the values of “x” defining the stoichiometries of the compositions. The values of “x” also define the crystalline structures of the compositions. Heightened piezoelectric responses occur for x&#39;s corresponding to tetragonal polycrystalline structures located near the morphotropic phase boundary (MPB). 
     FIG. 1 shows x-ray diffraction spectra  11 ,  12  of polycrystalline bodies with stoichiometries described by (PMN) (1−x)  (PT) x . The spectra  11 ,  12  depend on x-ray scattering angle θ and the stoichiometric parameter “x”. The value of “x” that corresponds to the MPB is identifiable from x-ray diffraction spectra  11 ,  12 . For values of “x” on one side of the MPB, a split peak  14  exists in the x-ray pattern  11 . The split peak  14  indicates the presence of tetragonal domains in the polycrystalline composition. For values of “x” on the other side of the MPB, the corresponding peak  15  in the x-ray pattern  12  is no longer split. The presence of the non-split peak  15  indicates a rhombohedral phase. 
     FIG. 2 shows piezoelectric responses of various polycrystalline compositions whose stoichiometries are described by (PMN) (1−x)  (PT) x . The piezoelectric responses of the compositions are measured by values of a coefficient d 33  defined by d 33 =dP 3 /dE 3 . The coefficient d 33  defines the polarization P 3  produced along the direction of the applied electric field E 3 . The value of d 33  depends on the number “x” , which defines a composition&#39;s stoichiometry. 
     The compositions of (PMN) (1−x)  (PT) x  exhibit a peak  16  in coefficient d 33  near the morphotropic boundary (MPB) that separates different grain structures for the polycrystalline compositions. The MPB corresponds to a value of x in the range of about 0.33 to about 0.34. The maximum  17  of the peak  16  in the value of d 33  occurs for an “x” equal to about 0.35. 
     For values of x greater than about 0.32 and smaller than about 0.4, polycrystalline compositions of (PMN ) (1−x)  (PT) x , have larger coefficients d 33  than conventional polycrystalline ceramics of PZT, shown as PZT-A and PZT-B. In the range of “x≧0.35”, the compositions of (PMN) (1−x)  (PT) x  have tetragonal crystalline domain structures. 
     The unit cell of a tetragonal crystal is a parallelepiped in which one side has a different length than the other two sides. The side with the different length has different orientations in different crystalline domains. In such compositions, a piezoelectric response, in addition to the intrinsic contribution, is produced through realigning the various crystalline domains of the composition along the applied electric-field direction. For compositions near the MPB, individual tetragonal crystalline domains are more easily realigned by a poling operation than for compositions whose value of “x” places them farther away from the MPB. The poling operation includes applying an electric field to a composition. 
     For values of “x” greater than or equal to about 0.35, compositions of (PMN) (1−x)  (PT) x  have tetragonal polycrystalline structures. For values of “x” greater than about 0.32 and less than about 0.47, these compositions have larger coefficients d 33 , i.e., larger piezoelectric responses, than polycrystalline compositions based on BaTiO 3  or PbTiO 3 . 
     FIG. 3 shows a piezoelectric device  20  that includes a slab-shaped solid body  22  and gold electrodes  25 ,  26  located on opposite sides of the slab-shaped body  22 . The solid body  22  has a polycrystalline structure and a stoichiometric composition (PMN) (1−x)  (PT) x . The number “x” has a value greater than about 0.31 and smaller than about 0.47, and in preferred embodiments, “x” is greater than about 0.32 and less than about 0.40. The polycrystalline structure is formed by crystalline grains  23 ,  24  with different orientations and diameters smaller than about 10 microns. The solid body  22  has a piezoelectric response to applied electric fields. 
     In some embodiments, solid body  22  also includes a dopants that make up not more 2 percent of the weight of the solid body  22 . The presence of the dopants may increase the piezoelectric response of the solid body  22  as compared to an undoped body of the otherwise same composition. Exemplary dopants include lanthanum, cobalt, cobalt plus lanthanum, thorium, rhodium, and iridium. Combinations such as thorium plus iridium, indium plus manganese, sodium plus gallium, indium plus gallium, gallium plus iron, cobalt plus tantalum, and cobalt plus tungsten may also be used to dope solid body  22 . 
     The gold electrodes  25 ,  26  are used to apply a voltage across the width of the solid body  22 . 
     The solid body  22  has a permanent anisotropic axis “P” of maximal dielectric response. The axis “P” is perpendicular to the surfaces of electrodes  25 ,  26 . The piezoelectric response of the body  22  corresponds to a coefficient d 33  with a value greater than about 500 pico-coulombs per Newton (pC/N). In exemplary solid bodies  22 , the coefficient d 33  has a value of about 610 pC/N or larger. 
     FIG. 4 is a flow chart for a process  30  for fabricating a piezoelectric device  20  of FIG.  3 . Fabrication process  30  includes forming a sintering mixture of powdered MgNb 2 O 6 , powdered TiO 2 , and powdered PbO (step  32 ). In the sintering mixture, proportions are selected to produce a desired stoichiometric composition of solid body  22  after sintering. Multiple sinterings are performed on the mixture of oxide powders to produce polycrystalline solid body  22  in a selected shape (step  34 ). To compensate for evaporation of lead at the sintering temperatures, the mixture includes an additional amount of PbO powder. Performing high-temperature sintering steps in a closed container reduces vaporization of lead. 
     The sintering produces a solid body  22  that is described by a stoichiometric formula (PMN) (1−x)  (PT) x . The stoichiometric parameter “x” is greater than or equal to about 0.31 and smaller than or equal to about 0.47. For better piezoelectric responses, some exemplary fabrication processes form the solid body with a composition in which x is greater than about 0.32 and less than about 0.40. 
     After the sintering, metallic electrodes  25 ,  26  are formed on opposite surfaces of the solid body  22  to produce an electromechanical device (step  36 ). The electrodes  25 ,  26  are used to pole the solid body  22  with a strong electric field (step  38 ). The poling generates a permanent anisotropic polarizability along a direction perpendicular to the electrodes  25 ,  26  by permanently realigning crystalline domains in the solid body  22 . 
     An exemplary embodiment of the fabrication process  30  of FIG. 4 is described below. 
     To perform the exemplary embodiment, powdered MgNb 2 O 6  is made. To make the powdered MgNb 2 O 6 , powdered MgO is dried and combined with powdered Nb 2 O 5  to form a mixture with equal molar amounts of MgO and Nb 2 O 5 . Then, the mixture is sintered via a multi-step process to produce the MgNb 2 O 6 . The sintering process includes heating the mixture at a temperature of about 1100° C. for about 12 hours, grinding the mixture to a powder, and reheating the ground mixture at a higher temperature of about 1200° C. for about 24 hours. After the multi-step sintering, the produced solid is reground to produce the powdered MgNb 2 O 6 . 
     In the exemplary embodiment, powdered MgNb 2 O 6 , powdered PbO, and powdered TiO 2  are combined to form a mixture in which stoichiometry proportions are chosen to produce a desired composition for solid body  22  after sintering. To produce a (PMN) (1−x)  (PT) x  composition, the stoichiometric proportions of MgNb 2 O 6 , TiO 2 , and PbO are selected to be equal to (1−x)/3, x, and (1+ε). The number “ε” is an excess of PbO that is introduced into the mixture to compensate for Pb evaporation during subsequent sintering. 
     In the exemplary embodiment, a multi-step process that sinters the mixture of MgNb 2 O 6 , TiO 2 , and PbO to produce solid body  22 . The multi-step process includes a first sintering of the mixture at about 900° C. for about 12 hours. The multi-step process includes a second sintering of the mixture at about 950° C. for about 12 more hours. The multi-step process includes a third sintering of the mixture at about 1200° C. to about 1250° C. for about 5 hours. The third sintering step is carried out in closed alumina crucibles to reduce Pb evaporation during this high-temperature sintering step and to enable better control on the final composition of the piezoelectric material. The second and third sinterings are performed at temperatures that would otherwise cause evaporation loss of Pb and thus, a lower molar percentage of Pb in the sintered solid body  22 . 
     Even with a crucible that is cemented closed some evaporation of Pb occurs. To compensate for this evaporation of lead during sintering, the sintering mixture includes a stoichiometric excess “ε” of PbO. For the exemplary embodiment, the value of the excess ε is equal to about 0.05. 
     In the exemplary embodiment, electrodes  25 ,  26  are attached to the solid body  22  by performing a sputtering deposition of gold on opposite sides of the sintered body  22 . 
     In the exemplary embodiment, electrodes  25 ,  26  are used to pole solid body  22  so that a permanent electric polarizability is produced along a direction perpendicular to the surfaces of electrodes  25 ,  26 . The poling is done by maintaining the solid body  22  at a temperature of about 130° C. while applying a strong electric field across the electrodes  25 ,  26 . The poling electric field has an intensity of about 7 to about 10 kilo-volts (kV) per centimeter (cm) and is applied across the electrodes  25 ,  26  for about 30 minutes. At the elevated poling temperature, the electric field causes crystalline domains in the solid body  22  to partially realign along the poling direction. After cooling the poled body  22  to room temperature, a permanent anisotropic polarizability remains. 
     Referring again to FIG. 3, the piezoelectric device  20  is used as a functional component of a variety of electromechanical devices. Some of these electromechanical devices are described below. 
     FIG. 5 shows a portion of an inkjet print head  50  that includes an array of ink jets  51 ,  51 ′ located on a support structure  49 . The ink jets  51 ,  51 ′ include piezoelectric devices  52 ,  52 ′ that have constructions analogous to that of device  20  of FIG.  3 . The ink jets  51 ,  51 ′ include ink reservoirs  53 ,  53 ′ with ink nozzles  54 ,  54 ′. The ink reservoirs  53 ,  53 ′ have flexible back surfaces adjacent to the piezoelectric devices  52 ,  52 ′. Movements of the piezoelectric devices  52 ,  52 ′ operate the ink jets  51 ,  51 ′ by deforming the ink reservoirs  53 ,  53 ′ and causing droplets of ink  55 ,  55 ′ to be squirted from nozzles  54 ,  54 ′ towards target spots on a sheet  56  of paper. The piezoelectric devices  52 ,  52 ′ are controlled by external voltage sources (not shown). 
     FIG. 6 shows a micro-electro-mechanical (MEM) device  60 . The MEM device  60  includes a supporting substrate  61 , a flexible structure  62  connected to the substrate  61 , and a piezoelectric device  63  having a construction analogous to that of device  20  of FIG.  3 . The piezoelectric device  63  rests on the substrate  61  and mechanically couples to the flexible structure  62 . The piezoelectric device  63  deforms in response to voltages applied to electrodes  65 ,  66  and functions as an actuator of the structure  62 . The deformations of the piezoelectric device  63  cause the structure  62  to bend thereby changing the orientation of mirror surface  67  located thereon. 
     FIG. 7 is a cross-sectional view of a pressure gage  70  that includes a moving sensor element  71  and a piezoelectric device  72  located on a support structure  74 , e.g., the piezoelectric device  20  of FIG.  3 . The mechanical element  71  moves vertically in response to pressure changes in chamber  73 . Vertical movements of the mechanical element  71  cause deformations of the piezoelectric device  72 . The deformations produce a voltage across electrodes  75 ,  76 . The voltage is measured by a voltmeter  78  to determine the pressure in the chamber  73 . 
     From the disclosure, drawings, and claims, other embodiments of the invention will be apparent to those skilled in the art.