Abstract:
A piezoelectric/electrostrictive element includes a laminated vibrator made of laminations of a piezoelectric/electrostrictive film and an electrode film, with the electrode film having an internal electrode film. The piezoelectric/electrostrictive element has a coating formed on a part of a surface of the laminated vibrator so that the coating selectively and completely covers a defect that would otherwise be exposed on the surface of the vibrator before the coating coats the defect. The defect extends to the internal electrode film and the remainder of the surface of the laminated vibrator is free of the coating.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a divisional of U.S. application Ser. No. 12/367,701, filed Feb. 9, 2009, and claims the benefit of Japanese Application Serial No. 2008-033035, filed Feb. 14, 2008, the entireties of which are incorporated herein. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to a piezoelectric/electrostrictive element with improved moisture resistance while having less degradation in its piezoelectric/electrostrictive properties, and a method of manufacturing such a piezoelectric/electrostrictive element. 
     BACKGROUND OF THE INVENTION 
     Piezoelectric/electrostrictive actuators have the advantage of precise displacement control of the order of submicrons. In particular, piezoelectric/electrostrictive actuators employing a sintered piezoelectric/electrostrictive ceramic body as a piezoelectric/electrostrictive body have the advantages of in addition to precise displacement control, high electromechanical conversion efficiency, high generative power, fast response speed, great durability, and low power consumption. Making use of these advantages, the piezoelectric/electrostrictive actuators are used for equipment such as inkjet printer heads and diesel engine injectors. 
     The piezoelectric/electrostrictive actuators employing a sintered piezoelectric/electrostrictive ceramic body as a piezoelectric/electrostrictive body, however, may at times suffer from the problem of a reduction in the amount of displacement at high humidities, regardless of the fact that there is no such problem at typical or ordinary humidity levels. The cause of such a reduction in the amount of displacement is considered because when a piezoelectric/electrostrictive actuator is polarized or repeatedly driven, stress is concentrated on where mechanical strength is low; such as at the grain boundary or in pores of a sintered piezoelectric/electrostrictive ceramic body, thereby forming microcracks or other defects, and subsequent possible water invasion into those defects may produce a conductive path, which consequently reduces the intensity of an electric field applied to a piezoelectric/electrostrictive film. 
     To prevent such a reduction in the amount of displacement at high humidities, it is effective to form a coating for covering microcracks or other defects, on the surface of a laminated vibrator made of laminations of a piezoelectric/electrostrictive film and an electrode film. 
     For example, Japanese Patent No. 3552013 describes a technique for improving moisture resistance by forming a coating (insulator layer 13) on the surface of a laminated vibrator (piezoelectric vibrator). Japanese Patent Application Laid-open No. 2007-175989 describes another technique for improving moisture resistance by forming a coating (protective film 100) on the surface of a laminated vibrator (piezoelectric vibrator 300). 
     However, although moisture resistance is improved by the formation of a coating on the surface of a laminated vibrator, the conventional techniques still have the problem of a reduced amount of displacement of a piezoelectric/electrostrictive actuator because the coating will restrain the laminated vibrator. To relax this problem, Japanese Patent Application Laid-open No. 2007-175989 has proposed that part of the coating be made of a pliant material (see paragraph [0051]); however, such a measure is insufficient to produce a satisfactory effect. 
     Note that this is not only the problem with piezoelectric/electrostrictive actuators but also the problem common to all piezoelectric/electrostrictive elements that include a laminated vibrator made of laminations of a piezoelectric/electrostrictive film and an electrode film. 
     SUMMARY OF THE INVENTION 
     According to a first aspect of the present invention, a method of manufacturing a piezoelectric/electrostrictive element including a laminated vibrator made of laminations of a piezoelectric/electrostrictive film and an electrode film includes the following steps: (a) bringing an electrodeposition coating fluid containing a coating component into contact with the laminated vibrator; and (b) selectively electrodepositing a coating material, which is to be a coating, on a defect exposed on a surface of the laminated vibrator and reaching a first electrode film of the laminated vibrator. 
     Since the coating covers the defect extended from the surface to first electrode film of the laminated vibrator, the moisture resistance of the piezoelectric/electrostrictive element is improved. In addition, the selective formation of the coating on the surface of the laminated vibrator thereby reduces degradation in the piezoelectric/electrostrictive properties of the piezoelectric/electrostrictive element due to the presence of the coating. 
     According to a second aspect of the present invention, a piezoelectric/electrostrictive element includes a laminated vibrator made of laminations of a piezoelectric/electrostrictive film and an electrode film; and a coating selectively covering a defect that is exposed on a surface of said laminated vibrator and reaches an electrode film of said laminated vibrator. 
     Since the coating covers the defect extended from the surface to first electrode film of the laminated vibrator, the moisture resistance of the piezoelectric/electrostrictive element is improved. In addition, the selective formation of the coating on the surface of the laminated vibrator additionally reduces degradation in the piezoelectric/electrostrictive properties of the piezoelectric/electrostrictive element due to the presence of the coating. 
     It is thus an object of the present invention to provide a piezoelectric/electrostrictive element that improves its moisture resistance while reducing degradation in its piezoelectric/electrostrictive properties. 
     These and other objects, features, aspects and advantages of the present invention will become more apparent from the following, detailed description of the present invention when taken in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a sectional view of a piezoelectric/electrostrictive element manufactured by a method of manufacturing a piezoelectric/electrostrictive element according to a first preferred embodiment. 
         FIG. 2  is a sectional view of another example of the piezoelectric/electrostrictive element. 
         FIG. 3  is an enlarged schematic view of a portion A in  FIG. 1 . 
         FIG. 4  is a flow chart for explaining the method of manufacturing a piezoelectric/electrostrictive element according to the first preferred embodiment. 
         FIG. 5  is a schematic view of an electrodeposition machine used in the method of manufacturing a piezoelectric/electrostrictive element according to the first preferred embodiment. 
         FIG. 6  is a schematic view of another example of the electrodeposition machine. 
         FIG. 7  is a schematic view of still another example of the electrodeposition machine. 
         FIG. 8  is a flow chart for explaining an electrodeposition process for producing a coating material according to a second preferred embodiment. 
         FIG. 9  is a schematic view of an electrodeposition machine used in the electrodeposition process for producing a coating material according to the second preferred embodiment. 
         FIG. 10  is a flow chart for explaining an electrodeposition process for producing a coating material according to a third preferred embodiment. 
         FIG. 11  is a schematic view of an electrodeposition machine used in the electrodeposition process for producing a coating material according to the third preferred embodiment. 
         FIG. 12  is a sectional view of a piezoelectric/electrostrictive element manufactured by a same manufacturing method as the method of manufacturing a piezoelectric/electrostrictive element according to first to third preferred embodiments. 
         FIG. 13  is an enlarged schematic view of a portion B in  FIG. 12 . 
         FIG. 14  is a schematic view of an electrodeposition machine used in manufacturing a piezoelectric/electrostrictive element according to a fourth preferred embodiment by a manufacturing method similar to the method of manufacturing a piezoelectric/electrostrictive element according to the first preferred embodiment. 
         FIG. 15  is a schematic view of an electrodeposition machine used in manufacturing a piezoelectric/electrostrictive element according to the fourth preferred embodiment by a manufacturing method similar to the method of manufacturing a piezoelectric/electrostrictive element according to the second preferred embodiment. 
         FIG. 16  is a schematic view of an electrodeposition machine used in manufacturing a piezoelectric/electrostrictive element according to the fourth preferred embodiment by a manufacturing method similar to the method of manufacturing a piezoelectric/electrostrictive element according to the third preferred embodiment. 
         FIG. 17  is a table showing pass rates for the amount of flexural displacement and for insulation resistance. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     &lt;1. First Preferred Embodiment&gt; 
     &lt;1-1 Structure of Piezoelectric/Electrostrictive Element  10 &gt; 
     {Entire Structure} 
       FIG. 1  diagrammatically illustrates a piezoelectric/electrostrictive element  10  manufactured by a method of manufacturing a piezoelectric/electrostrictive element according to a first preferred embodiment of the present invention.  FIG. 1  is a sectional view of the piezoelectric/electrostrictive element  10 . The piezoelectric/electrostrictive element  10  in  FIG. 1  forms the major part of an inkjet actuator used in an inkjet printer head. 
     As illustrated in  FIG. 1 , the piezoelectric/electrostrictive element  10  has a structure in which a laminated vibrator  110  is fixedly attached to the upper surface of a substrate  102  above a hollow or cavity  136 . The term “securely attached” refers to a connection of the laminated vibrator  110  to the substrate  102  by means of solid-phase reaction at the interface between the substrate  102  and the laminated vibrator  110 , without the use of any organic or inorganic adhesive. 
     {Substrate  102 } 
     The substrate  102  has a structure in which a base plate  106  and a diaphragm  108  are laminated from bottom to top in the order mentioned and integrated into a single unit. The substrate  102  is an insulator structure. There is no limitation on the type of the insulator, but in terms of heat resistance, chemical stability, and electric insulation, the substrate  102  should preferably be a sintered ceramic body containing at least one component selected from the group consisting of zirconium oxide, aluminum oxide, magnesium oxide, mullite, aluminum nitride, and silicon nitride. In particular, a sintered ceramic body of stabilized zirconium oxide is more preferable in terms of mechanical strength and toughness. The “stabilized zirconium oxide” herein refers to zirconium oxide in which crystal phase transition is suppressed by the addition of a stabilizer, and it includes not only stabilized zirconium oxide but also partially stabilized zirconium oxide. 
     The base plate  106  has a structure in which the cavity  136  with a long, narrow rectangular plane configuration is fowled in a plate of approximately uniform thickness. The diaphragm  108  is a plate of approximately uniform thickness. The diaphragm  108  should preferably have a thickness between 0.5 and 200 μm. This is because the thickness below this range tends to cause damage to the diaphragm  108 , while the thickness above this range tends to increase the stiffness of the diaphragm  108 , thus reducing the amount of flexural displacement of the piezoelectric/electrostrictive element  10 . 
     The substrate  102  is prepared by, for example, pressing and firing green sheets of an insulating ceramic. 
     As a substitute for the substrate  102 , a substrate  702  having a structure in which a base plate  704  having an ink jet hole  738  formed therein is further laminated under a base plate  704  and a diaphragm  708  which are similar respectively to the base plate  106  and the diaphragm  108  may be used as illustrated in the schematic view of a piezoelectric/electrostrictive element  70  in  FIG. 2 . 
     {Laminated Vibrator  110 } 
     The laminated vibrator  110  has a structure in which an electrode film  112 , a piezoelectric/electrostrictive film  114 , another electrode film  116 , another piezoelectric/electrostrictive film  118 , and another electrode film  120  are laminated from bottom to top in the order mentioned. 
     The electrode films  112 ,  116 , and  120  are films made of a conductor. There is no limitation on the type of the conductor, but in terms of electrical resistance and heat resistance, the electrode films  112 ,  116 , and  120  should preferably be a metal such as platinum, palladium, rhodium, gold, or silver; or an alloy containing such a metal as the main component. In particular, platinum with excellent heat resistance, or an alloy containing platinum as the main component is more preferable. 
     The electrode films  112 ,  116 , and  120  should preferably have a thickness between 0.1 and 15 μm. This is because the thickness above this range tends to increase the stiffness of the electrode films  112 ,  116 , and  120 , thus reducing the amount of flexural displacement of the piezoelectric/electrostrictive element  10 , while the thickness below this range tends to increase the electrical resistances of the electrode films  112 ,  116 , and  120 . 
     The electrode films  112 ,  116 , and  120  may be formed by applying either a paste where a conductive material is dispersed in a dispersion medium or a solution where resinate as a conductive material dissolves in a solvent and then by firing a resultant conductive material film after removal of the dispersion medium or the solvent. Or, they may be formed by deposition of a conductive material. The application of a paste is made by screen printing or any other similar technique, and the application of a solution is made by spin coating, spraying, or any other similar technique. The deposition of a conductive material is made by sputtering, resistance heating, or any other similar technique. Of course, these are only just examples of the method of formation, and other methods may be employed. 
     The piezoelectric/electrostrictive films  114  and  118  are films made of a piezoelectric/electrostrictive body. There is no limitation on the type of the piezoelectric/electrostrictive body, but in terms of electric-field-induced strains, the piezoelectric/electrostrictive films  114  and  118  should preferably be a sintered ceramic body of lead (Pb)-based perovskite oxide, and more preferably be a sintered ceramic body of lead zirconate titanate (Pb(Zr x Ti 1-x )O 3 ) or of lead zirconate titanate into which a simple oxide, a complex pevroskite oxide, or the like has been introduced. In particular, the piezoelectric/electrostrictive films  114  and  118  should more preferably be a sintered ceramic body containing nickel oxide (NiO) introduced in a solid solution of lead zirconate titanate and lead magnesium niobate (Pb(Mg 1/3 Nb 2/3 )O 3 ), or a sintered ceramic body of a solid solution of lead zirconate titanate and lead nickel niobate (Pb(Ni 1/3 Nb 2/3 )O 3 ). 
     The piezoelectric/electrostrictive films  114  and  118  should preferably have a thickness between 0.2 and 50 μm. This is because the thickness below this range tends to result in insufficient densification of the piezoelectric/electrostrictive films  114  and  118 , while the thickness above this range tends to increase the shrinkage stress of the piezoelectric/electrostrictive films  114  and  118  during sintering, thus requiring an increase in the thickness of the diaphragm  108 . 
     The piezoelectric/electrostrictive films  114  and  118  are formed by applying a paste where a piezoelectric/electrostrictive material is dispersed in a dispersion medium and then by firing a resultant piezoelectric/electrostrictive material film after removal of the dispersion medium. The application of a paste is made by screen printing or any other similar technique. Alternatively, the piezoelectric/electrostrictive films  114  and  118  may be formed by immersing a work-in-process into a slurry where a piezoelectric/electrostrictive material is dispersed in a dispersion medium to thereby induce electrophoresis of the piezoelectric/electrostrictive material toward an electrode film and then by firing a resultant piezoelectric/electrostrictive material film. Of course, these are only just examples of the method of formation, and other methods may be employed. 
     The electrode films  112  and  116  are opposed to each other with the piezoelectric/electrostrictive film  114  therebetween, and the electrode films  116  and  120  are opposed to each other with the piezoelectric/electrostrictive film  118  therebetween. While  FIG. 1  illustrates the case where the laminated vibrator  110  includes two layers of the piezoelectric/electrostrictive films  114  and  118 , the laminated vibrator may include three or more layers of piezoelectric/electrostrictive films. A laminated vibrator including three or more layers of piezoelectric/electrostrictive films has a structure in which a piezoelectric/electrostrictive film and an electrode film are alternately laminated one above another. In this case, the lowermost or uppermost layer of the laminated vibrator may be an inactive piezoelectric/electrostrictive film that is not sandwiched by electrode films so that no electric field is applied. The present invention is also applicable to the case where a laminated vibrator includes only a single piezoelectric/electrostrictive film and has electrode films formed on both main surfaces of the piezoelectric/electrostrictive film. 
     While the major part of the electrode film  112  is situated between the substrate  102  and the piezoelectric/electrostrictive film  114 , the electrode film  112  has its one end extending outside the area where the cavity  136  is formed and thus making a feeder  142  for giving a drive signal. While the major part of the electrode film  116  is situated between the piezoelectric/electrostrictive films  114  and  118 , the electrode film  116  has its one end extending from between the piezoelectric/electrostrictive films  114  and  118  to the outside of the area where the cavity  136  is formed and thus making a feeder  144  for giving a drive signal. The electrode films  112  and  120  are electrically short-circuited by an electrode film  122  formed on the end faces of the piezoelectric/electrostrictive films  114  and  118 . In the following description, these electrically short-circuited electrode films  112 ,  120 , and  122  are referred to as an “external electrode film  132 ”, and the electrode film  116  that is not electrically short-circuited to the external electrode film  132  as an “internal electrode film  134 .” 
     {Coating  128 } 
       FIG. 3  is an enlarged schematic view of a portion A in  FIG. 1 . As illustrated in  FIG. 3 , the piezoelectric/electrostrictive film  118  possesses a large number of microcracks and other defects (hereinafter referred to simply as “defects”)  152 . Some of the defects (hereinafter referred to as “surface-exposed defects”)  154  are exposed on the surface of the laminated vibrator  110  and reaches the internal electrode film  134 . The piezoelectric/electrostrictive element  10  includes coatings  128  that selectively cover the surface-exposed defects  154 . The positions, sizes, number, and the like of surface-exposed defects  154  vary in each piezoelectric/electrostrictive element  10 , and so do the positions, sizes, number, and the like of coatings  128  in each piezoelectric/electrostrictive element  10 . The formation of the coatings  128  on the surface of the laminated vibrator  110  prevents moisture invasion into the surface-exposed defects  154 , thereby preventing the formation of a conductive path that connects the surface of the laminated vibrator  110  and the internal electrode film  134 . This improves the moisture resistance of the piezoelectric/electrostrictive element  10 . In addition, the selective formation of the coatings  128  on the surface of the piezoelectric/electrostrictive element  10  reduces degradation in the piezoelectric/electrostrictive properties of the piezoelectric/electrostrictive element  10  due to the presence of the coatings  128 . The coatings  128  are films made of an insulator. The coatings  128  are formed by electro depositing a coating material on the surface-exposed defects  154  exposed on the surface of the laminated vibrator  110  and then by subjecting the laminated vibrator  110  to post treatment. 
     {Operation of Piezoelectric/Electrostrictive Element  10 } 
     In the configuration described above, when a drive signal is fed between the feeders  142  and  144  and an electric field is applied to the piezoelectric/electrostrictive films  114  and  118 , the piezoelectric/electrostrictive films  114  and  118  are expanded and contracted in a direction perpendicular to the direction of lamination, which causes a bending of the united diaphragm  108  and laminated vibrator  110 . With this bending, the piezoelectric/electrostrictive element  10  will press ink in the cavity  136 . &lt;1-2. Method of Manufacturing Piezoelectric/Electrostrictive Element  10 &gt; 
       FIG. 4  is a flow chart for explaining a method of manufacturing a piezoelectric/electrostrictive element according to the first preferred embodiment.  FIG. 5  is a schematic view of an electrodeposition machine used in the method of manufacturing a piezoelectric/electrostrictive element according to the first preferred embodiment. 
     {Preparation of Laminated Structure  100 } 
     As illustrated in  FIG. 4 , in the manufacture of a piezoelectric element, a laminated structure  100  is first prepared by fixedly attaching the laminated vibrator  110  to the upper surface of the substrate  102  (in step S 101 ). 
     {Growth of Defects} 
     Then, direct voltage is applied between the feeders  142  and  144  to polarize the piezoelectric/electrostrictive films  114  and  118  (in step S 102 ), and a drive signal is applied between the feeders  142  and  144  to drive the laminated vibrator  110  (in step S 103 ). Such polarization and drive are not an absolute necessity prior to the immersion of the laminated structure  100  in an electrodeposition coating fluid  164 , but the polarization and drive in advance will allow advance growth of defects, which might be generated afterward, and advance covering of such defects with the coatings  128 . This further improves the moisture resistance of the piezoelectric/electrostrictive element  10 . The process for growing defects in advance should preferably include both the polarization and the drive; however it may include only either one of the polarization and the drive. Alternatively, instead of or in addition to the polarization and the drive, a heat shock test in which the laminated structure  100  is alternately exposed to high and low temperatures, or any other similar process may be performed. Still alternatively, this process for growing defects in advance may be omitted. 
     {Surface Treatment} 
     The laminated vibrator  110  is then subjected to surface treatment for improving the adhesion of the coatings  128  to the surface of the laminated vibrator  110  (in step S 104 ). Performing the surface treatment before immersion of the laminated structure  100  in the electrodeposition coating fluid  164  will improve the adhesion of the coatings  128  to the surface of the laminated structure  100 , thus further improving the moisture resistance of the piezoelectric/electrostrictive element  10 . The surface treatment for improving the adhesion of coatings to the surface of the laminated vibrator  110  includes the process for removing an organic compound adhering to the surface of the laminated vibrator  110  by plasma-cleaning, the process for forming a self-organizing film on the surface of the laminated vibrator  110 , and the like. Alternatively, the surface treatment may be performed prior to the process for growing defects. Still alternatively, the surface treatment may be omitted. 
     {Electrodeposition of Coating Material} 
     Then, as illustrated in  FIG. 5 , with the external electrode film  132  electrically short-circuited to a counter electrode  162  provided separately from the laminated vibrator  110 , the entire laminated structure  100  and the counter electrode  162  are immersed in the electrodeposition coating fluid  164  containing a coating component to bring the electrodeposition coating fluid  164  into contact with the surfaces of the laminated vibrator  110  and the counter electrode  162  (in step S 105 ). 
     The counter electrode  162  is a flat plate made of platinum. Of course, the counter electrode  162  may be made of a metal other than platinum. Being a flat plate is not an absolute necessity for the counter electrode  162 . Thus, a counter electrode  762  which is a bending plate as illustrated in  FIG. 6 , or a counter electrode  862  which is a coil that can house the laminated structure  100  therein as illustrated in  FIG. 7  may be used as a substitute for the counter electrode  162 . 
     The electrodeposition coating fluid  164  may be either a solution where a coating component is dissolved in a solvent or a fluid dispersion where a coating component is dispersed in a dispersion medium. The electrodeposition coating fluid  164  may be either of a cation type where the coating component becomes positively charged or of an anion type where the coating component becomes negatively charged. Examples of the coating material include a carbon polymer compound such as an epoxy resin, a polyimide resin, a polyamide-imide resin, or acrylic resin; a silicon polymer compound such as a silicone resin; and nanoparticles of oxide such as alumina where a dispersing agent is absorbed and electrically charged on the surface. Examples of the solvent or the dispersion medium include an inorganic solvent such as water; and an organic solvent such as alcohol. Alternatively, the electrodeposition coating fluid  164  may contain a curing agent such as blocked isocyanate; or a catalyst such as a tin compound. After the laminated structure  100  and the counter electrode  162  are immersed in the electrodeposition coating fluid  164 , voltage is applied between the internal electrode film  134  and the counter electrode  162  to induce electrophoresis of the coating material toward the surface-exposed defects  154 , whereby the coating material is selectively electrodeposited on the surface-exposed defects  154  (in step S 106 ). When the electrodeposition coating fluid  164  is of the cation type, the internal electrode film  134  is connected to the negative pole of the power supply, and the counter electrode  162  to the positive pole. When the electrodeposition coating fluid  164  is of the anion type, the internal electrode film  134  is connected to the positive pole of the power supply, and the counter electrode  162  to the negative pole. The selective electrodeposition on the surface-exposed defects  154  is possible because the surface-exposed defects  154  make a conductive path so that an electric field formed between the internal electrode film  134  and the counter electrode  162  leaks out of the surface-exposed defects  154  into the electrodeposition coating fluid  164 , thereby causing the coating component to be drawn to the surface-exposed defects  154 . 
     Here, the external electrode film  132  is electrically short-circuited to the counter electrode  162  so that the external electrode film  132  has a potential equal to that of the counter electrode  162 . Thus, even if the major part of the external electrode film  132  is situated on the surface of the laminated structure  100  and in contact with the electrodeposition coating fluid  164 , the coating material is less prone to being adhered to the surface of the external electrode film  132 . This, however, does not make it an absolute necessity to make an electrical short circuit between the counter electrode  162  and parts of the electrode films  112 ,  116 ,  120 , and  120  of the laminated vibrator  110  so that those electrode films are connected to the same pole as the counter electrode  162 . That is, all the electrode films  112 ,  116 ,  120 , and  122  may be connected to the pole opposite to that to which the counter electrode  162  is connected. 
     After the electrodeposition of the coating material on the surface-exposed defects  154 , the laminated structure  100  and the counter electrode  162  are pulled up from the electrodeposition coating fluid  164  to remove the electrodeposition coating fluid  164  from the surface of the laminated vibrator  110  (in step S 107 ), and then the laminated structure  100  is separated from the counter electrode  162  (in step S 108 ). 
     {Post Treatment} 
     The laminated structure  100  separated from the counter electrode  162  is then subjected to post treatment so that the film of the coating material makes the ultimate coatings  128  (in step S  109 ). The post treatment includes the process for hardening the film of the coating material, the process for increasing the densification of the film of the coating material, the process for enhancing the adhesion of the film of the coating material to the surface of the laminated vibrator  110 , the process for removing an unnecessary part of the adhered coating material, and the like. For example when the coating material is a resin, it is preferable that polymerization reaction be caused by heating or light irradiation. When the coating material is nanoparticles of oxide, it is preferable that the coating material be sintered by firing. If a large amount of coating material has been adhered to the surface of the external electrode film  132  due to the absence of an electrical short circuit between the external electrode film  132  and the counter electrode  162 , the coating material should preferably be removed by mechanical polishing or the like. 
     &lt;2. Second Preferred Embodiment&gt; 
     A second preferred embodiment relates to another electrodeposition process of a coating material, which can be adopted as a substitute for the electrodeposition process for producing a coating material (steps S 105  to S 108 ) according to the first preferred embodiment. 
       FIG. 8  is a flow chart for explaining the electrodeposition process for producing a coating material according to the second preferred embodiment.  FIG. 9  is a schematic view of an electrodeposition machine  260  used in the electrodeposition process for producing a coating material according to the second preferred embodiment. 
     In the electrodeposition process for producing a coating material according to the second preferred embodiment, first of all, as illustrated in  FIG. 9 , with the external electrode film  132  electrically short-circuited to a counter electrode  262  provided separately from the laminated vibrator  110 , a droplet of an electrodeposition coating fluid  264  is placed on the laminated vibrator  110  and the counter electrode  262  is formed on that droplet, so that the electrodeposition coating fluid  264  is brought into contact with the surfaces of the laminated vibrator  110  and the counter electrode  262  (in step S 201 ). The counter electrode  262  and the electrodeposition coating fluid  264  may be the same as the counter electrode  162  and the electrodeposition coating fluid  164  used in the electrodeposition process of a coating material according to the first preferred embodiment. As described, the electrodeposition fluid  264  is brought into contact with only the portion of the surface of the laminated structure  100  that requires electrodeposition of the coating material. This prevents the coating material from being adhered to where the formation of the coatings  128  is unnecessary. 
     Thereafter, voltage is applied between the internal electrode film  134  and the counter electrode  262  to induce electrophoresis of a coating component toward the surface-exposed defects  154 , whereby the coating material is selectively electrodeposited on the surface-exposed defects  154  (in step S 202 ). The selective electrodeposition on the surface-exposed defects  154  is possible because the surface-exposed defects  154  make a conductive path so that an electric field formed between the internal electrode film  134  and-the counter electrode  262  leaks out of the surface-exposed defects  154  into the electrodeposition fluid  264 , thereby causing the coating component to be drawn to the surface-exposed defects  154 . After the electrodeposition of the coating material on the surface-exposed defects  154 , the electrodeposition coating fluid  264  is removed from the surface of the laminated vibrator  110  (in step S 203 ), and the laminated structure  100  is separated from the counter electrode  262  (in step S 204 ). 
     &lt;3. Third Preferred Embodiment&gt; 
     A third preferred embodiment relates to still another electrodeposition process of a coating material, which can be adopted as a substitute for the electrodeposition of producing a coating material (steps S 105  to S 108 ) according to the first preferred embodiment. 
       FIG. 10  is a flow chart for explaining the electrodeposition process of a coating material according to the third preferred embodiment.  FIG. 11  is a schematic view of an electrodeposition machine  360  used in the electrodeposition process for producing a coating material according to the third preferred embodiment. 
     In the electrodeposition process of a coating material according to the third preferred embodiment, first of all, a droplet of an electrodeposition coating fluid  364  is placed on the laminated vibrator  110  so as to bring the electrodeposition coating fluid  364  into contact with the surface of the laminated vibrator  110  (in step S 301 ). The electrodeposition coating fluid  364  may be the same as the electrodeposition coating fluid  164  used in the electrodeposition of producing a coating material according to the first preferred embodiment. As described, the electrodeposition fluid  364  is brought into contact with only the portion of the surface of the laminated structure  100  that requires electrodeposition of a coating material. This prevents the coating material from being adhered to where the formation of the coatings  128  is unnecessary. 
     Thereafter, voltage is applied between the internal electrode film  134  and the external electrode film  132  to induce electrophoresis of a coating component toward the surface-exposed defects  154 , whereby the coating material is selectively electrodeposited on the surface-exposed defects  154  (in step S 302 ). The selective electrodeposition on the surface-exposed defects  154  is possible because the surface-exposed defects  154  make a conductive path so that an electric field formed between the internal electrode film  134  and the external electrode film  132  leaks out of the surface-exposed defects  154  into the electrodeposition fluid  364 , thereby causing the coating component to be drawn to the surface-exposed defects  154 . After the electrodeposition of the coating material on the surface-exposed defects  154 , the electrodeposition coating fluid  364  is removed from the surface of the laminated vibrator  110  (in step S 303 ). 
     &lt;4. Fourth Preferred Embodiment&gt; 
     &lt;4-1. Structure of Piezoelectric/Electrostrictive Element  40 &gt; 
       FIG. 12  is a schematic view of a piezoelectric/electrostrictive element  40  manufactured by a manufacturing method similar to the methods of manufacturing a piezoelectric/electrostrictive element according to the first to third preferred embodiments of the present invention.  FIG. 12  shows a cross section of the piezoelectric/electrostrictive element  40 . The piezoelectric/electrostrictive element  40  in  FIG. 12  forms the major part of an inkjet actuator used in an inkjet printer head. As illustrated in  FIG. 12 , a laminated vibrator  410  of the piezoelectric/electrostrictive element  40  has a structure in which a piezoelectric/electrostrictive film  412 , an electrode film  414 , another piezoelectric/electrostrictive film  416 , another electrode film  418 , another piezoelectric/electrostrictive film  420 , another electrode film  422 , another piezoelectric/electrostrictive film  424 , another electrode film  426 , and another piezoelectric/electrostrictive film  428  are laminated one above another in the order mentioned. While  FIG. 12  illustrates the case where the laminated vibrator  410  includes five layers of the piezoelectric/electrostrictive films  412 ,  416 ,  420 ,  424 , and  428 , the number of piezoelectric/electrostrictive films of the laminated vibrator may be increased or reduced. The present invention is also applicable even to the case where a laminated vibrator includes only a single piezoelectric/electrostrictive film and has electrode films formed on both main surfaces of the piezoelectric/electrostrictive film. 
     The piezoelectric/electrostrictive films  412 ,  416 ,  420 ,  424 , and  428  and the electrode films  414 ,  418 ,  422 , and  426  can be formed of the same materials and by the same methods as the piezoelectric/electrostrictive films  114  and  118  and the electrode films  112 ,  116 , and  120  of the piezoelectric/electrostrictive element  10  according to the first preferred embodiment. 
     The electrode films  414  and  422  are exposed on one side of the laminated vibrator  410  and electrically short-circuited to each other by an electrode film  430  formed on that side. The electrode films  418  and  426  are exposed on the other side of the laminated vibrator  410  and electrically short-circuited to each other by an electrode film  432  formed on that side. Parts of the electrode films  430  and  432  make feeders  442  and  444 , respectively, for giving a drive signal. 
       FIG. 13  is an enlarged schematic view of a portion B in  FIG. 12 . As illustrated in  FIG. 13 , the piezoelectric/electrostrictive film  428  of the laminated vibrator  410  possesses defects  452 . Some of the defects, namely surface-exposed defects  454 , are exposed on the surface of the laminated vibrator  410  and reaches the electrode film  426 . The piezoelectric/electrostrictive element  40  includes coatings  429  that selectively cover the surface-exposed defects  454 . The positions, sizes, number, and the like of surface-exposed defects  454  vary in each piezoelectric/electrostrictive element  40 , and so do the positions, sizes, number, and the like of coatings  429  in each piezoelectric/electrostrictive element  40 . The formation of the coatings  429  on the surface of the laminated vibrator  410  prevents moisture invasion into the surface-exposed defects  454 , thereby preventing the formation of a conductive path that connects the surface of the laminated vibrator  410  and an internal electrode film  434 . This improves the moisture resistance of the piezoelectric/electrostrictive element  40 . 
     The coatings  429  are films made of an insulator. The coatings  429  are fowled by electrodepositing a coating material on the surface-exposed defects  454  exposed on the surface of the laminated vibrator  410  and then by subjecting the laminated vibrator  410  to post treatment. 
     &lt;Operation of Piezoelectric/Electrostrictive Element  40 &gt; 
     In the configuration described above, when a drive signal is fed between the feeders  442  and  444  and an electric field is applied to the piezoelectric/electrostrictive films  412 ,  416 ,  420 ,  424 , and  428 , the piezoelectric/electrostrictive films  412 ,  416 ,  420 ,  424 , and  428  are expanded and contracted in a direction perpendicular to the direction of lamination. With this expansion and contraction, the piezoelectric/electrostrictive element  40  can press ink. 
     {Manufacture of Piezoelectric/Electrostrictive Element  40 } 
     This piezoelectric/electrostrictive element  40  can also be manufactured by a manufacturing method similar to the methods of manufacturing a piezoelectric/electrostrictive element according to the first to third preferred embodiments.  FIGS. 14 to 16  are schematic views of electrodeposition machines  4602 ,  4604 , and  4606 , respectively, that are used in manufacturing the piezoelectric/electrostrictive element  40  according to the fourth preferred embodiment by a manufacturing method similar to the methods of manufacturing a piezoelectric/electrostrictive element according to the first to third preferred embodiments. As illustrated in  FIGS. 14 to 16 , in manufacturing the piezoelectric/electrostrictive element  40  by the manufacturing method similar to the methods of manufacturing a piezoelectric/electrostrictive element according to the first to third preferred embodiments, the feeders  442  and  444  are used respectively as substitutes for the feeders  142  and  144  of the piezoelectric/electrostrictive element  10 . 
     &lt;Modifications&gt; 
     The above description has given the methods of manufacturing a piezoelectric/electrostrictive element, taking an actuator as an example. Those manufacturing methods can also produce a piezoelectric/electrostrictive element other than an actuator, e.g., a sensor or a resonator, in a similar fashion, and can produce a piezoelectric/electrostrictive element in which surface-exposed defects are selectively covered with coatings. However, the effect of improving moisture resistance in adopting the method of manufacturing a piezoelectric/electrostrictive element according to the present invention is in particular noticeable in actuators, because the actuators usually produce significant deformation in piezoelectric/electrostrictive films and thus are likely to generate surface-exposed defects. 
     EXAMPLES 
     Example 1 
     In Example 1, the piezoelectric/electrostrictive element  10  was manufactured by the method of manufacturing a piezoelectric/electrostrictive element according to the first preferred embodiment. In Example 1, however, the surface treatment in step S 104  was omitted. 
     In Example 1, the substrate  102  was made of partially stabilized zirconium oxide; the electrode films  112  and  116  of platinum; the electrode film  120  of gold; and the piezoelectric/electrostrictive films  114  and  118  of a solid solution of lead zirconate titanate and lead nickel niobate. The electrodeposition coating fluid  164  was of an aqueous cation type, in which a coating component was an epoxy resin. Electrophoretic conditions for electrodeposition of the coating material were a temperature of 25° C., an applied voltage of 400 V, and a voltage application time of 20 seconds. Further, post treatment involved cleaning; 15-minute preliminary drying at 100° C.; and subsequent hardening of the epoxy resin by ultraviolet irradiation. This produced the coatings  128  having a thickness of 0.3 μm. 
     As to the resultant piezoelectric/electrostrictive element  10 , the laminated vibrator  110  was driven at 40° C. and at ordinary humidity of 55% to measure the amount of flexural displacement with a laser Doppler displacement meter and the insulation resistance with an insulation testing set. Thereafter, the laminated vibrator  110  was driven at 40° C. and at high humidity of 85% to measure the amount of flexural displacement and the insulation resistance in a similar fashion, to thereby check the pass rates therefor. The results were tabulated in  FIG. 17 . 
     Example 2 
     In Example 2, the piezoelectric/electrostrictive element  40  was manufactured by a manufacturing method similar to the method of manufacturing a piezoelectric/electrostrictive element according to the first preferred embodiment. In Example 2, the surface treatment in step S 104  was omitted, and the electrode films  430  and  432  were connected in a unit to the positive pole of the power supply, instead of being connected to the counter electrode  162 . 
     In Example 2, the piezoelectric/electrostrictive films  412 ,  416 ,  420 ,  424 , and  428  were made of an alloy of silver and palladium; and the piezoelectric/electrostrictive films  412 ,  416 ,  420 ,  424 , and  428  of a solid solution of lead zirconate titanate and lead nickel niobate. The electrodeposition coating fluid  164  was of an aqueous anion type, in which a coating component was an polyimide resin. Electrophoretic conditions for electrodeposition of the coating material were a temperature of 25° C., an applied voltage of 400 V, and a voltage application time of 20 seconds. The post treatment involved cleaning; 15-minute preliminary drying at 100° C.; and subsequent hardening of the polyimide resin by 30-minute heating at 210° C. This produced the coatings  429  having a thickness of 0.3 μm. In addition, after the formation of the coatings  429 , the coating material adhered to the surface of the electrode films  430  and  432  were removed by mechanical polishing in Example 2. 
     The resultant piezoelectric/electrostrictive element  40  was measured in the same manner as in Example 1 for the amount of flexural displacement and for the insulation resistance to check the pass rates therefor. The results were tabulated in  FIG. 17 . 
     Example 3 
     In Example 3, the piezoelectric/electrostrictive element  10  was manufactured in the same manner as in Example 1, except in that the coating component was alumina nanoparticles negatively charged in a carboxylic dispersant and that the post treatment involved two-hour firing at 900° C. in an electric furnace to sinter the alumina nanoparticles. This produced the coatings  128  having a thickness of 0.2 μm. The resultant piezoelectric/electrostrictive element  10  was measured in the same manner as in Example 1 for the amount of flexural displacement and for the insulation resistance to check the pass rates therefor. The results were tabulated in  FIG. 17 . 
     Example 4 
     In Example 4, the piezoelectric/electrostrictive element  10  was manufactured in the same manner as in Example 1, except in that the coating component was silica particulates and siloxane oligomer containing a methyl group; that the post treatment involved 15-minute heat treatment at 120° C. to gelatinize a film of the coating material; and that the surface treatment in step S 104  was not omitted, i.e., performed. This produced the coatings  128  where silica particulates are dispersed in a gelled film. The resultant piezoelectric/electrostrictive element  10  was measured in the same manner as in Example 1 for the amount of flexural displacement and for the insulation resistance to check the pass rates therefor. The results were tabulated in  FIG. 17 . 
     Comparative Example 1 
     A piezoelectric/electrostrictive element was manufactured in the same manner as in Example 1, except in that steps S 103  and S 105  to S 109  were omitted. 
     The resultant piezoelectric/electrostrictive element was measured in the same mariner as in Example 1 for the amount of flexural displacement and for the insulation resistance to check the pass rates therefor. The results were tabulated in  FIG. 17 . 
     Comparative Example 2 
     A piezoelectric/electrostrictive element was manufactured in the same manner as in Example 2, except in that steps S 103  and S 105  to S 109  were omitted. The resultant piezoelectric/electrostrictive element was measured in the same manner as in Example 1 for the amount of flexural displacement and for the insulation resistance to check the pass rates therefor. The results were tabulated in  FIG. 17 . 
     Comparison between Examples and Comparative Examples 
     As shown in  FIG. 17 , under ordinary temperature conditions, any of Examples 1 to 4 within the scope of the present invention and any of Comparative Examples 1 and 2 outside the scope of the present invention showed high pass rates for both the amount of flexural displacement and the insulation resistance. However, under high temperature conditions, although Examples 1 to 4 within the scope of the present invention showed high pass rates for both the amount of flexural displacement and the insulation resistance, Comparative Examples 1 and 2 showed low pass rates therefor. 
     While the invention has been shown and described in detail, the foregoing description is in all aspects illustrative and not restrictive. It is therefore understood that numerous modifications and variations can be devised without departing from the scope of the invention. In particular, it goes without saying that any combination of the techniques described in the first to fourth preferred embodiments will be apparent to those skilled in the art.