Patent Publication Number: US-2011063348-A1

Title: Liquid Ejection Head, Methods of Manufacturing and Driving the Same, and Image Recording Apparatus

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a liquid ejection head, methods of manufacturing and driving the same, and an image recording apparatus, and particularly, relates to a liquid ejection head, methods of manufacturing and driving the same, and an image recording apparatus capable of obtaining high generated pressure. 
     2. Description of the Related Art 
     It is known that, in a piezoelectric liquid ejection head, an individual electrode of a piezoelectric actuator is formed into a circle along a rim part of a pressure chamber. Japanese Patent Application Publication No. 2006-150948 discloses a liquid ejection head that uses such a circular individual electrode and improves the displacement efficiency by forming a piezoelectric body in the area other than the region corresponding to the central section of a pressure chamber. 
     Japanese Patent Application Publication No. 2004-42329, on the other hand, discloses a liquid ejection head that uses a general individual electrode that is not in a circular form. In the liquid ejection head, a diaphragm is previously deflected into a projecting shape protruding toward a pressure chamber so that sufficient displacement can be obtained even when the diaphragm is formed into a thin film, and the diaphragm is displaced to the pressure chamber side to eject liquid. 
     Japanese Patent Application Publication No. 2000-141643 and International Publication No. WO 01/072521 each disclose a liquid ejection head that uses a general individual electrode that is not in a circular form. In the liquid ejection head, a diaphragm is previously deflected into a projecting shape protruding toward the side opposite to a pressure chamber so that sufficient displacement can be obtained even when the diaphragm is formed into a thin film, and the diaphragm is displaced to the pressure chamber side to eject liquid. 
     Japanese Patent No. 4287278 discloses a liquid ejection head in which a piezoelectric body is disposed on top of, and in contact with, a diaphragm that is deflected into a projecting shape protruding toward the side opposite to a pressure chamber, an individual electrode and common electrode are disposed on this piezoelectric body, a driving voltage is applied between the individual electrode and the common electrode to displace the diaphragm to the pressure chamber side, and thereby liquid droplets are ejected. 
     However, a disadvantage of the liquid ejection head that is configured as described in Japanese Patent Application Publication No. 2006-150948 is that the rigidity thereof decreases as a result of forming the piezoelectric body without including the area corresponding to the central section of the pressure chamber, and, for example, highly viscous liquid might not be ejected. 
     A disadvantage of the liquid ejection head that is configured as described in Japanese Patent Application Publication No. 2004-42329 is that a high displacement cannot be ensured because the diaphragm is further displaced in a deflected direction. 
     A disadvantage of the liquid ejection heads that are configured as described in Japanese Patent Application Publication No. 2000-141643 and International Publication No. WO 01/072521 is that a high displacement cannot be obtained because the diaphragm is displaced from the state where it is deflected into a projecting shape protruding toward the side opposite to the pressure chamber, to the state where it is brought back to a neutral point with no deflection. Another disadvantage of the liquid ejection heads described in Japanese Patent Application Publication No. 2000-141643 and International Publication No. WO 01/072521 is that it is difficult to control the thickness of the diaphragm and therefore its thickness fluctuation because the diaphragms of these liquid ejection heads are formed by means of a film formation. 
     A disadvantage of the liquid ejection head that is configured as described in Japanese Patent No. 4287278 is that high voltage is required to drive this liquid ejection head in order to obtain a significant displacement due to the special electrode configuration. 
     SUMMARY OF THE INVENTION 
     The present invention has been contrived in view of such circumstances, and an object thereof is to provide a liquid ejection head including a piezoelectric actuator that has a highly rigid and produces high displacement, methods of manufacturing and driving such a liquid ejection head, and an image recording apparatus. 
     In order to attain an object described above, one aspect of the present invention is directed to a liquid ejection head comprising a piezoelectric actuator that changes volume of a pressure chamber to cause liquid in the pressure chamber to eject from a nozzle connected with the pressure chamber, wherein the piezoelectric actuator is formed into a projecting shape protruding toward the pressure chamber, and displaced in a direction opposite to the pressure chamber when applied with a driving voltage, to increase the volume of the pressure chamber. 
     According to this aspect of the invention, the piezoelectric actuator is formed into a projecting shape protruding toward the pressure chamber and displaced in a direction opposite to the pressure chamber when the driving voltage is applied. 
     Desirably, the piezoelectric actuator passes beyond a neutral point with no deflection and deforms into a projecting shape protruding toward the direction opposite to the pressure chamber, when applied with the driving voltage. 
     According to this aspect of the invention, when the driving voltage is applied, the piezoelectric actuator passes beyond the neutral point with no deflection and deforms into a projecting shape protruding toward the side opposite to the pressure chamber. As a result, a high displacement amount can be obtained. 
     Desirably, the piezoelectric actuator includes: a diaphragm which forms a wall surface of the pressure chamber; a piezoelectric body which is disposed on a surface of the diaphragm opposite from the pressure chamber; an individual electrode which is disposed on one surface of the piezoelectric body and in a region where the individual electrode overlaps with a rim part of the pressure chamber; and a common electrode which is disposed on another surface of the piezoelectric body. 
     According to this aspect of the invention, the individual electrode is disposed in the form of a circle in a region where the individual electrode overlaps with the rim part of the pressure chamber. In the piezoelectric actuator having the individual electrode formed into a circle, a tensile stress is generated by applying the driving voltage. Therefore, the displacement can be increased. 
     Desirably, the piezoelectric actuator is formed into the projecting shape protruding toward the pressure chamber by forming the piezoelectric body on the diaphragm having a coefficient of linear expansion lower than that of the piezoelectric body, according to a thin film forming method involving a heat treatment. 
     According to this aspect of the invention, the piezoelectric actuator is formed into a projecting shape protruding toward the pressure chamber by forming the piezoelectric body on the diaphragm having a coefficient of linear expansion lower than that of the piezoelectric body, by means of a thin film forming method involving a heat treatment. In this manner, the piezoelectric actuator that is bent into a projecting shape protruding toward the pressure chamber can be obtained easily. 
     Desirably, the piezoelectric body contains Nb, and a coefficient of linear expansion of the piezoelectric body is made higher than that of the diaphragm by adjusting an additive amount of the Nb contained in the piezoelectric body. 
     According to this aspect of the invention, the additive amount of Nb is adjusted and the coefficient of linear expansion of the piezoelectric body is thereby made higher than the coefficient of linear expansion of the diaphragm. As a result, the coefficient of linear expansion of the piezoelectric body can be adjusted easily, and a desired piezoelectric actuator can readily be configured. 
     Desirably, the diaphragm is made of silicon. 
     According to this aspect of the invention, the diaphragm is made of silicon with a low coefficient of linear expansion. 
     Desirably, the pressure chamber is formed in a silicon substrate. 
     Desirably, the pressure chamber and the nozzle are provided in plurality, respectively, in such a manner that the pressure chambers are arrayed in a staggered manner in a substrate, and the nozzles are arrayed in a staggered manner in a nozzle surface. 
     According to this aspect of the invention, the pressure chambers are arrayed in a staggered manner and the nozzles are arrayed in a staggered manner on the nozzle surface. In other words, the pressure chambers and the nozzles are arrayed two-dimensionally in a first direction, as well as in a second direction that is inclined at a predetermined angle to the first direction. Accordingly, the density of the nozzles can be increased. 
     Desirably, the liquid ejection head further comprises a control device which controls application of the driving voltage to the piezoelectric actuator so as to adjust ejection of a liquid droplet from the nozzle in such a manner that the driving voltage is applied to the piezoelectric actuator only when the liquid droplet is ejected from the nozzle. 
     According to this aspect of the invention, the driving voltage is applied to the piezoelectric actuator only when a liquid droplet is ejected from the nozzle. Therefore, the liquid ejection head can be driven highly reliably. 
     Desirably, the piezoelectric body is polarized in terms of a thickness direction of the piezoelectric body, and contracts in a direction perpendicular to the thickness direction when the driving voltage is applied to the piezoelectric body. 
     Desirably, the individual electrode has an outer circumference corresponding to an outer circumference of the piezoelectric body, and has an inner circumference similar to the outer circumference of the individual electrode. 
     In order to attain an object described above, another aspect of the present invention is directed to a method of manufacturing a liquid ejection head comprising a piezoelectric body disposed on a diaphragm forming a wall surface of a pressure chamber, the method comprising the step of forming the piezoelectric body on the diaphragm having a coefficient of linear expansion lower than that of the piezoelectric body, according to a thin film forming method involving a heat treatment, in such a manner that the piezoelectric body and the diaphragm are bent into a projecting shape protruding toward the pressure chamber. 
     According to this aspect of the invention, the piezoelectric body is formed on the diaphragm having a coefficient of linear expansion lower than that of the piezoelectric body, by means of a thin film forming method involving a heat treatment. By this means, the piezoelectric actuator that is bent into a projecting shape protruding toward the pressure chamber can be obtained easily. 
     Desirably, the method of manufacturing a liquid ejection head further comprises the step of forming an individual electrode in a region where the individual electrode overlaps with a rim part of the pressure chamber. 
     According to this aspect of the invention, the individual electrode is disposed in the form of a circle in a region where the individual electrode overlaps with the rim part of the pressure chamber. According to the piezoelectric actuator having the individual electrode formed into a circle, a tensile stress is generated by applying the driving voltage. Therefore, a piezoelectric actuator that produces high displacement can be configured. 
     Desirably, the diaphragm is made of silicon. 
     According to this aspect of the invention, the diaphragm is made of silicon with a low coefficient of linear expansion. 
     Desirably, the pressure chamber is formed in a silicon substrate. 
     Desirably, an additive of Nb is adjusted to control the coefficient of linear expansion of the piezoelectric body. 
     According to this aspect of the invention, the additive amount of Nb is adjusted and the coefficient of linear expansion of the piezoelectric body is thereby made higher than the coefficient of linear expansion of the diaphragm. 
     Desirably, the piezoelectric body is polarized in terms of a thickness direction of the piezoelectric body, and contracts in a direction perpendicular to the thickness direction when a driving voltage is applied to the piezoelectric body. 
     Desirably, the individual electrode has an outer circumference corresponding to an outer circumference of the piezoelectric body, and has an inner circumference similar to the outer circumference of the individual electrode. 
     In order to attain an object described above, another aspect of the present invention is directed to an image recording apparatus comprising any of the liquid ejection heads defined above. 
     According to this aspect of the invention, image recording is carried out by using any of the liquid ejection heads described above. 
     In order to attain an object described above, another aspect of the present invention is directed to a method of driving any of the liquid ejection heads defined above, comprising the step of applying a driving voltage to the piezoelectric actuator only when a liquid droplet is ejected from the nozzle. 
     According to this aspect of the invention, the driving voltage is applied to the piezoelectric actuator only when a liquid droplet is ejected from the nozzle. Therefore, the liquid ejection head can be driven highly reliably. 
     According to the present invention, a highly rigid piezoelectric actuator that produces high displacement can be obtained. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram showing the entire configuration of an inkjet recording apparatus; 
         FIG. 2  is a plane perspective view of an ink ejection surface of an inkjet head; 
         FIG. 3  is a vertical cross-sectional diagram showing a part of an inkjet head; 
         FIG. 4  is a plan view of a piezoelectric actuator; 
         FIGS. 5A and 5B  are explanatory diagrams illustrating operations of a piezoelectric actuator; and 
         FIG. 6  is a diagram showing an example of a drive waveform of driving voltage applied to a piezoelectric actuator. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Hereinafter, preferred embodiments of the present invention are described with reference to the accompanying drawings. 
     Note that an example in which the present invention is applied to an inkjet head is described herein. First of all, a configuration of an inkjet head recording apparatus using such an inkjet head is described. 
     Configuration of an Inkjet Recording Apparatus 
       FIG. 1  is a diagram showing the entire configuration of an inkjet recording apparatus for printing an image on a sheet of paper by means of an inkjet method. 
     An inkjet recording apparatus  10  includes a paper supply unit  20  for supplying a sheet (piece of paper)  14 , a treatment liquid application unit  30  for applying a predetermined treatment liquid to a print surface of the sheet  14 , a rendering unit  40  for depositing ink droplets from an inkjet head onto the print surface of the sheet  14  to render an image, a dryer  50  for drying the ink deposited on the sheet  14 , a fixing unit  60  for fixing the image rendered on the sheet  14 , and a paper discharging unit  70  for discharging the sheet after printing it. 
     The treatment liquid application unit  30 , the rendering unit  40 , the dryer  50 , and the fixing unit  60  are provided with impression cylinders (conveyance drums)  34 ,  44 ,  54  and  64  as conveyance devices, respectively. The sheet  14  is wrapped around the circumferential surfaces of these impression cylinders  34 ,  44 ,  54 ,  64  and conveyed in the treatment liquid application unit  30 , rendering unit  40 , dryer  50 , and fixing unit  60  while rotating. 
     Transfer cylinders (conveyance drums)  32 ,  42 ,  52  and  62  serving as conveyance devices are disposed between the paper supply unit  20  and the treatment liquid application unit  30 , between the treatment liquid application unit  30  and the rendering unit  40 , between the rendering unit  40  and the dryer  50 , as well as between the dryer  50  and the fixing unit  60 . The sheet  14  is wrapped around the circumferential surfaces of these transfer cylinders  32 ,  42 ,  52 ,  62  and conveyed in each space between the units while rotating. 
     The impression cylinders  34 ,  44 ,  54 ,  64  and the transfer cylinders  32 ,  42 ,  52 ,  62  are disposed alternately and driven by motors that are not shown, to rotate in directions opposite to each other. In other words, the impression cylinders  34 ,  44 ,  54 ,  64  are rotated in a counterclockwise direction in  FIG. 1 , while the transfer cylinders  32 ,  42 ,  52 ,  62  are rotated in a clockwise direction in  FIG. 1 . 
     Note that the circumferential surfaces of the impression cylinders  34 ,  44 ,  54 ,  64  and the transfer cylinders  32 ,  42 ,  52 ,  62  are each provided with grippers G for gripping a leading end of the sheet  14 . The sheet  14  is wrapped about each of the circumferential surfaces of the impression cylinders  34 ,  44 ,  54 ,  64  and of the transfer cylinders  32 ,  42 ,  52 ,  62  while being gripped by the grippers G at the leading end part of the sheet  14 . 
     Note that the sheet  14  is wrapped around the circumferential surfaces of the impression cylinders  34 ,  44 ,  54 ,  64  with its image recording surface (i.e. print surface) on the outside, and is wrapped around the circumferential surfaces of the transfer cylinders  32 ,  42 ,  52 ,  62  with the rear surface (which is a surface opposite from the print surface) on the outside. 
     The sheet  14  that is supplied by the paper supply unit  20  is delivered to the impression cylinder  34  of the treatment liquid application unit  30  via the transfer cylinder  32 , and is then delivered from the impression cylinder  34  of the treatment liquid application unit  30  to the impression cylinder  44  of the rendering unit  40  via the transfer cylinder  42 . The sheet  14  is then delivered from the impression cylinder  44  of the rendering unit  40  to the impression cylinder  54  of the dryer  50  via the transfer cylinder  52 , and is then delivered from the impression cylinder  54  of the dryer  50  to the impression cylinder  64  of the fixing unit  60  via the transfer cylinder  62 . The sheet  14  is further transferred from the impression cylinder  64  of the fixing unit  60  to the paper discharging unit  70 . In this series of conveyance processes, the sheet  14  passes through the treatment liquid application unit  30 , the rendering unit  40 , the dryer  50  and the fixing unit  60 , is then subjected to a required process at each unit, and thereby an image is formed on the print surface (image recording surface). 
     The configuration of each of the units (the paper supply unit  20 , the treatment liquid application unit  30 , the rendering unit  40 , the dryer  50 , the fixing unit  60 , and the paper discharging unit  70 ) of the inkjet recording apparatus  10  of the present embodiment is described hereinafter. 
     Paper Supply Unit 
     The paper supply unit  20  has a paper supply apparatus  22  and a paper tray  24  for continuously supplying sheets (coated paper for printing, for example)  14  one by one. 
     The paper supply apparatus  22  supplies, to the paper tray  24 , the sheets  14  that are stored in a stacked state in a stacker that is not shown, one by one sequentially from the top. 
     The paper tray  24  sends the sheets  14  that are sequentially supplied one by one from the paper supply apparatus  22 , toward the transfer cylinder  32 . 
     The sheets  14  that are sent out from the paper tray  24  are delivered to the impression cylinder  34  of the treatment liquid application unit  30  via the transfer cylinder  32 . 
     Treatment liquid Application Unit 
     The treatment liquid application unit  30  applies a predetermined treatment liquid to the print surface of the sheet  14 . The treatment liquid application unit  30  includes the impression cylinder (treatment liquid drum)  34  for conveying the sheet  14 , and a treatment liquid application apparatus  36  for applying the predetermined treatment liquid to the print surface (image recording surface) of the sheet  14  conveyed by the treatment liquid drum  34 . 
     The treatment liquid drum  34  receives the sheet  14  from the transfer cylinder  32  (by gripping the leading end of the sheet  14  using the grippers G), wraps the sheet  14  around the circumferential surface thereof, and rotates and conveys the sheet  14 . In this mechanism, the treatment liquid drum  34  receives the sheet  14  from the transfer cylinder  32 , with the print surface of the sheet  14  on the outside, and rotates and conveys the sheet  14 . 
     The treatment liquid application apparatus  36  applies the predetermined treatment liquid to the print surface of the sheet  14  that is rotated and conveyed by the treatment liquid drum  34 . The treatment liquid application apparatus  36  presses an application roller of which the circumferential surface is provided with the treatment liquid, so as to contact with the circumferential surface of the sheet  14 , applying the treatment liquid to the print surface of the sheet  14 . 
     Here, as the treatment liquid to be applied to the sheet  14 , a liquid that functions to react with ink deposited by the rendering unit  40  so as to aggregate the color materials of the ink is used. When such treatment liquid is deposited in advance and the ink droplets are deposited to the sheet  14 , the color materials of the ink droplets are aggregated immediately after the ink droplets land, and thus the color materials can be prevented from being mixed even when the ink droplets land adjacent to each other. 
     The treatment liquid application unit  30  is configured as described above. The print surface of the sheet  14  that is delivered from the transfer cylinder  32  to the treatment liquid drum  34  is given the treatment liquid by the treatment liquid application apparatus  36  in the course of being rotated and conveyed by the treatment liquid drum  34 . Then, the sheet  14  applied with the treatment liquid is delivered from the treatment liquid drum  34  to the transfer cylinder  42  and then delivered from the transfer cylinder  42  to the impression cylinder  44  of the rendering unit  40 . 
     Rendering Unit 
     The rendering unit  40  deposits the ink droplets in C, M, Y, K colors to the print surface of the sheet  14 , to form a color image on the print surface of the sheet  14 . This rendering unit  40  includes the impression cylinder (recording drum)  44  for conveying the sheet  14 , and inkjet heads  46 C,  46 M,  46 Y,  46 K for depositing the ink droplets in C, M, Y, K colors onto the sheet  14 . 
     The recording drum  44  receives the sheet  14  from the transfer cylinder  42 , wraps the sheet  14  around the circumferential surface thereof, and rotates and conveys the sheet  14 . In this mechanism, the recording drum  44  receives the sheet  14  from the transfer cylinder  42 , with the print surface of the sheet  14  on the outside, and rotates and conveys the sheet  14 . 
     When receiving the sheet  14  from the treatment liquid drum  34  of the treatment liquid application unit  30 , the transfer cylinder  42  receives the sheet  14  from the treatment liquid drum  34 , with the other side (rear surface) opposite to the image recording surface (print surface) on the outside, and rotates and conveys the sheet  14 . 
     The four inkjet heads  46 C,  46 M,  46 Y,  46 K, which are disposed around the recording drum  44  at regular intervals, eject the ink droplets in the corresponding colors toward the recording drum  44 . The inkjet heads  46 C,  46 M,  46 Y,  46 K are configured by line heads corresponding to the width of the sheet. A row of nozzles having the length corresponding to the width of the sheet is formed on a surface (nozzle surface) of each inkjet head facing the recording drum  44 , along a direction perpendicular to a conveyance direction of the sheet  14 . 
     The configurations of the inkjet heads  46 C,  46 M,  46 Y,  46 K and the method for driving the same are described below in detail. 
     The rendering unit  40  is configured as described above. The sheet  14  that is delivered from the treatment liquid drum  34  to the recording drum  44  via the transfer cylinder  42  passes under the inkjet heads  46 C,  46 M,  46 Y,  46 K while being rotated and conveyed by the recording drum  44 . The ink droplets in C, M, Y, K colors are deposited on the print surface by the inkjet heads  46 C,  46 M,  46 Y,  46 K during the passage of the sheet  14 , whereby the color image is recorded on the print surface. 
     At this process, because the treatment liquid that has a function for aggregating the color materials of the ink is applied to the sheet  14  in advance, the color materials can be prevented from being mixed, so that a high quality image can be recorded. 
     Note in the present embodiment that a water-based ink having thermoplastic resins dispersed therein is used as the ink ejected from each of the inkjet heads  46 C,  46 M,  46 Y and  46 K. 
     The sheet  14 , of which the ink droplets in C, M, Y, K colors from the inkjet heads  46 C,  46 M,  46 Y,  46 K has been ejected and thereby an image is recorded onto the print surface, is delivered from the recording drum  44  to the transfer cylinder  52 , and then from the transfer cylinder  52  to the impression cylinder  54  of the dryer  50 . 
     Dryer 
     The dryer  50  dries the sheet  14  on which the image is recorded. The dryer  50  includes the impression cylinder (drying drum)  54  for conveying the sheet  14 , and a drying apparatus  56  that performs a drying process on the sheet  14  conveyed by the drying drum  54 . 
     The drying drum  54  receives the sheet  14  from the transfer cylinder  52 , wraps the sheet  14  around the circumferential surface thereof, and rotates and conveys the sheet  14 . In this mechanism, the drying drum  54  receives the sheet  14  from the transfer cylinder  52 , with the print surface of the sheet  14  on the outside, and rotates and conveys the sheet  14 . The drying apparatus  56  performs a process of evaporating the moisture present on the sheet. In other words, when the ink is deposited on the sheet  14  by the rendering unit  40 , a liquid component of the ink and a liquid component of the treatment liquid that are separated by the aggregation reaction between the treatment liquid and the ink remain on the sheet, and therefore, the drying apparatus  56  performs the process of evaporating and removing the liquid components remaining on the sheet. This drying apparatus  56  evaporates and removes the liquid components present on the sheet, by blowing warm air to the sheet  14  conveyed by the drying drum  54 . 
     The dryer  50  is configured as described above. The sheet  14  that is delivered from the recording drum  44  to the drying drum  54  via the transfer cylinder  52  is subjected to the drying process in which the warm air is blown from the drying apparatus  56  to the sheet  14  while the sheet  14  is conveyed by the drying drum  54 . The sheet  14  that passes through the drying apparatus  56  is delivered from the drying drum  54  to the transfer cylinder  62  and conveyed to the fixing unit  60 . 
     Fixing Unit 
     The fixing unit  60  heats and pressurizes the sheet  14  to fix the image rendered to the print surface. This fixing unit  60  includes the impression cylinder (fixing drum)  64  for conveying the sheet  14 , and a heat roller  66  for performing a heating/pressurizing process on the sheet  14  that is conveyed by the fixing drum  64 . 
     The fixing drum  64  receives the sheet  14  from the transfer cylinder  62 , wraps the sheet  14  around the circumferential surface thereof, and rotates and conveys the sheet  14 . In this mechanism, the fixing drum  64  receives the sheet  14  from the transfer cylinder  62 , with the print surface of the sheet  14  on the outside, and rotates and conveys the sheet  14 . 
     The heat roller  66  heats and pressurizes the ink that is dried by the dryer  50 , so as to weld the thermoplastic resins dispersed in the ink so that a film of the ink is formed. The heat roller  66  also straightens cockles formed on the sheet  14  at the same time. This heat roller  66  is formed so as to correspond to the width of the sheet and heated to a predetermined temperature by an embedded heat source (infrared heater, for example). A pressurizing device which is not shown presses the heat roller  66  toward the circumferential surface of the fixing drum  64 , with a predetermined pressure. 
     The fixing unit  60  is configured as described above. The sheet  14  that is delivered from the transfer cylinder  62  to the fixing drum  64  is heated and pressurized as the heat roller  66  is pressed and brought into contact with the print surface of the sheet  14  while the sheet  14  is conveyed by the fixing drum  64 . As a result, the thermoplastic resins dispersed in the ink are adhered (weld), forming the ink into a film. In addition, the cockles formed on the sheet  14  are straightened at the same time. 
     The sheet  14  that is heated and pressed by the heat roller  66  is delivered from the fixing drum  64  to the paper discharging unit  70 . 
     Paper Discharging Unit 
     The paper discharging unit  70  recovers the sheets  14  into a stacker  72  after a series of image recording steps are performed on the sheets  14 . The paper discharging unit  70  has a conveyor  74  that conveys the sheets  14  to the stacker  72 . The sheets  14  that are subjected to the fixing process by the fixing unit  60  are delivered from the fixing drum  64  to the conveyor  74 . The sheets  14  are then conveyed by the conveyor  74 , to the position where the stacker  72  is set. The stacker  72  is set at a predetermined recovery position, and the sheets  14  conveyed by the conveyor  74  are discharged into the stacker  72 , sequentially stacked in the stacker  72 , and recovered. 
     Printing Operations 
     Next, printing operations performed the inkjet recording apparatus  10  are described. 
     The paper supply apparatus  22  supplies the sheets  14  stored in the stacker (not shown) one by one sequentially from the top to the paper tray  24 . The sheets  14  that are supplied to the paper tray  24  are delivered to the treatment liquid drum  34  of the treatment liquid application unit  30  via the transfer cylinder  32 . Then, the treatment liquid is applied by the treatment liquid application apparatus  36  to the surface of each of the sheets  14  while each of the sheets  14  is conveyed by the treatment liquid drum  34 . 
     Each sheet  14  applied with the treatment liquid is delivered from the treatment liquid drum  34  to the rendering drum  44  of the rendering unit  40  via the transfer cylinder  42 . The ink droplets in corresponding colors are deposited from the inkjet heads  46 C,  46 M,  46 Y,  46 K to the sheet  14  while the sheet  14  is conveyed by the rendering drum  44 , whereby the image is formed on the print surface. 
     The sheet  14  having the image formed on the print surface thereof is delivered from the rendering drum  44  to the drying drum  54  of the dryer  50  via the transfer cylinder  52 . The warm air is blown from the drying apparatus  56  to the sheet  14  while the sheet  14  is conveyed by the drying drum  54 , whereby the ink deposited onto the print surface of the sheet  14  is dried. 
     The sheet  14  having the dried ink is delivered from the drying drum  54  to the fixing drum  64  via the transfer cylinder  62 . Then, the heat roller  76  is pressed and brought into contact with the print surface of the sheet  14  while the sheet  14  is conveyed by the fixing drum  64 , whereby the ink is heated and pressurized. As a result, the image formed on the print surface of the sheet  14  is fixed. 
     The sheet  14  having the image fixed thereto by the fixing unit  60  is delivered to the conveyor  74  of the paper discharging unit  70 , conveyed to the stacker  72  by the conveyor  74 , and then discharged into the stacker. 
     As described above, the printing is carried out through the series of steps where paper supply, treatment liquid application, image rendering, ink drying, image fixation, and paper discharge are performed in this order. 
     Configurations of the Inkjet Heads 
     Next, the configurations of the inkjet heads  46 C,  46 M,  46 Y,  46 K in the above-described inkjet recording apparatus  10  are described. 
     Because the structures of the inkjet heads  46 C,  46 M,  46 Y,  46 K corresponding to the colors are all the same, reference numeral  100  is used to illustrate the representative inkjet head. 
       FIG. 2  is a plane perspective view of an ink ejection surface of an inkjet head  100 . 
     As shown in  FIG. 2 , in the inkjet head  100  of the present embodiment, the nozzles  110  are disposed in a staggered manner on an ink ejection surface  102 . In other words, the plurality of nozzles  110  are arrayed two-dimensionally at predetermined intervals in a longitudinal direction of the head (a first direction) and a direction that is inclined at a predetermined angle to the longitudinal direction (a second direction). By arraying the nozzles  110  in such a staggered manner, the substantial space between the nozzles that is projected in the longitudinal direction of the head (the direction perpendicular to the sheet conveyance direction) can be narrowed, and the density of the nozzles  110  can be increased. 
     The nozzles  110  are connected with respective pressure chambers  112  through nozzle flow paths  114 . The pressure chambers  112  also are arrayed in a staggered manner on a surface parallel to the ink ejection surface  102  in the same manner as the nozzles  110 . 
     As shown in  FIG. 2 , the planar shape of each pressure chamber  112  is formed into an oval, and the long axis (the axis in the longitudinal direction) thereof is disposed parallel to the longitudinal direction of the inkjet head  100 . A nozzle flow path  114  and an individual supply flow path  116  are connected with each end of this pressure chamber  112  in the longitudinal direction thereof 
     The nozzle flow paths  114  are formed to extend vertically downward from the pressure chambers  112  (see  FIG. 3 ) and are connected with the nozzles  110  formed on the ink ejection surface  102 . 
     Each individual supply flow path  116 , on the other hand, is connected with a common supply flow path  118  that supplies the ink to each of the pressure chambers  112 . As shown in  FIG. 2 , this common supply flow path  118  is configured by a main flow part  118   a  that extends in a direction parallel to the longitudinal direction of the inkjet head  100 , and a plurality of branching flow parts  118   b  that branch from the main flow part  118   a  and extend in the second direction. The individual supply flow paths  116  that are connected with the pressure chambers  112  respectively are connected with the branching flow parts  118   b  respectively. The ink is supplied from an ink tank, not shown, to the ink supply port  120  formed on one end of the main flow part  118   a.  The ink that is supplied to the ink supply port  120  is then supplied from the main flow part  118   a  to each of the pressure chambers  112  via each branching flow part  118   b  and individual supply flow path  116 . 
       FIG. 3  is a vertical cross-sectional diagram showing a part of the inkjet head  100 . 
     As shown in the diagram, the inkjet head  100  of the present embodiment has a structure in which a nozzle plate  130 , a flow path substrate  132 , and a piezoelectric actuator  134  are stacked sequentially. 
     The nozzle plate  130  is a substrate in which the nozzles  110  are formed, and is joined to the flow path substrate  132  so as to cover the lower surface of the flow path substrate  132 . 
     In the inkjet head  100  of the present embodiment, the nozzle plate  130  is made of silicon (Si) using a SOI (Silicon On Insulator) substrate. In so doing, first, the nozzles are formed in the SOI substrate by means of anisotropic etching or the like, and the SOI substrate having the nozzles formed therein is joined to the lower surface of the flow path substrate  132 . Then, after joining the SOI substrate and the flow path substrate  132 , the Si substrate and SiO 2  layer of the SOI substrate are removed. As a result, the nozzle plate  130  made of Si is formed on the lower surface of the flow path substrate  132 . The flow path substrate  132  is a substrate in which the pressure chambers  112 , the nozzle flow paths  114 , the individual supply flow paths  116 , the common supply flow path  118  and the like are formed. In the inkjet head  100  of the present embodiment, this flow path substrate  132  is formed from a Si substrate having a predetermined thickness. The pressure chambers and the like are formed by subjecting this Si substrate to etching processing or the like. 
     A piezoelectric actuator  134  is provided correspondingly to each of the pressure chambers  112  formed in the flow path substrate  132  and is configured mainly by a diaphragm  136  and a piezoelectric element  138  provided on the diaphragm  136 . 
     The diaphragm  136  is joined to the flow path substrate  132  so as to cover an upper surface of the flow path substrate  132 . A ceiling surface (one wall surface) of each pressure chamber  112  formed in the flow path substrate  132  is configured by joining the diaphragm  136  to the upper surface of the flow path substrate  132 . In other words, an upper part of each pressure chamber  112  formed in the flow path substrate  132  is opened, and the diaphragm  136  is joined thereto so that the opened upper part is covered, whereby the ceiling surface is formed. 
     In the inkjet head  100  of the present embodiment, this diaphragm  136  is made of Si and formed using a SOI substrate. In so doing, the SOI substrate is joined to the upper surface of the flow path substrate  132  with the surface Si layer at the bottom. After joining the SOI substrate to the upper surface of the flow path substrate  132 , the Si substrate and SiO 2  layer of the SOI substrate are removed. As a result, the diaphragm  136  made of Si is formed on the upper surface of the flow path substrate  132 . 
     Furthermore, in the inkjet head  100  of the present embodiment, the diaphragm  136  is bent into a projecting shape protruding toward the pressure chamber  112  side in the region where the pressure chamber  112  is formed (the region where the ceiling surface of the pressure chamber  112  is formed), as shown in  FIG. 3 . 
     A piezoelectric element  138  is provided with respect to each of the pressure chambers  112  and disposed on the diaphragm  136  configuring the ceiling surface of each pressure chamber  112 . Each piezoelectric element  138  is configured by a piezoelectric body  140 , a lower electrode  142  serving as a common electrode, and an upper electrode  144  serving as an individual electrode, wherein the piezoelectric body  140  is sandwiched between the lower electrode  142  and the upper electrode  144 . As shown in  FIG. 4 , the planar shape of the piezoelectric element  138  is formed into an oval to correspond to the planar shape (oval shape) of the pressure chamber  112 , and is also bent into a projecting shape protruding toward the pressure chamber, as with the diaphragm  136 . 
     The piezoelectric body  140  is formed into an oval to correspond to the planar shape of the pressure chamber  112 , and disposed coaxially with the pressure chamber  112 . This piezoelectric body  140  is formed to be larger than the external form of the pressure chamber  112  and disposed on the diaphragm such that an outer circumferential part of the piezoelectric body  140  projects from the rim part of the pressure chamber  112  by a predetermined distance. 
     This piezoelectric body  140  is made of a piezoelectric material with ferroelectricity, such as lead zirconium titanate (PZT) or other ceramic materials. 
     The lower electrode  142  is formed over the entire upper surface of the diaphragm  136 . The piezoelectric body  140  is formed on an upper surface of this lower electrode  142 . 
     This lower electrode  142  is made of a conductive material such as gold, silver, copper, palladium, platinum, and titanium and formed on the diaphragm  136  by means of a screen printing method, a sputtering method, an evaporation method, or the like. 
     The lower electrode  142  is connected to the ground through a flexible printed wiring (not shown). 
     On the other hand, an upper electrode  144  is formed into a circle on the upper surface of each piezoelectric body  140  so as to overlap with the rim part of each pressure chamber  112 . The outer circumference of the upper electrode  144  is formed into an oval corresponding to the outer circumference of the piezoelectric body  140 , and the inner circumference of the same is formed into a similar oval thereto. In other words, the upper electrode  144  is formed to cover the region other than the center of the pressure chamber  112 . 
     The upper electrodes  144  are made of a conductive material, such as gold, silver, copper, palladium, platinum, and titanium, and formed on the piezoelectric body  140  individually by means of a screen printing method, a sputtering method, an evaporation method, or the like. 
     Each upper electrode  144  is connected to a drive circuit (not shown) via a flexible printed wiring (not shown), and a driving voltage is selectively applied to each upper electrode  144  via this driving circuit. 
     As described above, in the inkjet head  100  of the present embodiment, the piezoelectric actuator  134  is formed to be bent into a projecting shape protruding toward the pressure chamber in the initial state. The piezoelectric actuator  134  that is bent into a projecting shape protruding toward the pressure chamber in the initial state as described above can be formed as follows. 
     Specifically, when forming the piezoelectric body  140  on the diaphragm  136  made of Si via the lower electrode  142 , a PZT film of the piezoelectric body  140  is formed by a thin film forming method (a sputtering method, sol-gel method, laser abrasion method, CDV method etc., for example), and a heat treatment is performed thereon during this formation process. 
     The coefficient of linear expansion of the PZT is greater than that of Si forming the base substrate and therefore generates a downward projecting shape of PZT. Consequently, in the initial state, the piezoelectric actuator  134  that is bent into a projecting shape protruding toward the pressure chamber can be formed. For example, the coefficient of linear expansion of Si is 2.6E-6/° C., and the coefficient of linear expansion of genuine PZT is 3.0E-6/° C. In this case, a PZT film is formed at 500° C. or 400° C., and when the temperature returns to a room temperature (25° C.), a structure that bends into a downward projecting shape by approximately 0.1 μm (0.08 μm) can be formed in the piezoelectric actuator having a width of 240 μm (where the thickness of the PZT is 2.5 μm and the thickness of the diaphragm is 11.5 μm). At this moment, when a voltage of 25 V is applied, the structure is displaced into a projecting shape protruding above by 137 nm (according to simulation performed by Ansys). 
     In the inkjet head  100  of the present embodiment, a thin film of the piezoelectric body  140  is formed on the Si diaphragm  136  via the lower electrode  142  therebetween, by means of a sputtering process involving a heat treatment. 
     Operations of the Inkjet Head 
     The operations of the inkjet head  100  are described next. 
     In the inkjet head  100 , the volume of each of the pressure chambers  112  is increased or reduced by individually applying a driving voltage to the upper electrode  144  of a piezoelectric actuator  134  provided correspondingly to each pressure chamber  112 , and ink droplets are ejected from the nozzles  110  connected with the respective pressure chambers  112 . 
     As described above, the drive circuit (not shown) applies the driving voltage to each piezoelectric actuator  134 . 
     As shown in  FIG. 5A , the piezoelectric actuator  134  is bent into a projecting shape protruding toward the pressure chamber in the initial state (in the state where the driving voltage is not applied). 
     When the driving voltage is applied from the drive circuit to the upper electrode  144  of the piezoelectric actuator  134 , the circular region of the piezoelectric body  140  sandwiched between the circular upper electrode  144  and lower electrode  142  contracts in a direction (horizontal direction) perpendicular to a polarization direction (thickness direction of the piezoelectric body  140 ). As the circular region of the piezoelectric body  140  contracts in the direction perpendicular to the polarization direction, the diaphragm  136  passes beyond the neutral point with no deflection (a state with no vertical concave/convex shape) and deforms into a projecting shape protruding toward the side opposite to the pressure chamber  112  as shown in  FIG. 5B . 
     In this case, because the piezoelectric actuator  134  is previously bent into a projecting shape protruding toward the pressure chamber, application of the driving voltage allows the piezoelectric actuator  134  to deform easily. In other words, because the piezoelectric body  140  receives a tensile stress by previously being bent into a projecting shape protruding toward the pressure chamber, applying the driving voltage and acting the tensile force allows the piezoelectric actuator  140  to deform easily using the buckling effect. 
     Then, by deforming the diaphragm  136  into a projecting shape protruding toward the side opposite to the pressure chamber, the volume of each pressure chamber  112  increases. 
     Thereafter, when the application of the driving voltage to the upper electrode  144  is stopped, the diaphragm  136  returns to its original shape (the projecting shape that is bent to protrude toward the pressure chambers side). Consequently, the volume of the pressure chamber  112  decreases, and the ink inside the pressure chamber is pushed out to the nozzle flow path  114 , whereby an ink droplet is ejected from the nozzle  110 . 
     In the inkjet head  100  of the present embodiment, the driving voltage is applied to the piezoelectric actuator  134  that is previously bent into a projecting shape protruding toward the pressure chamber, to displace the diaphragm  136  toward the side opposite to the pressure chamber. Then, the shape of the diaphragm  136  is returned to its original shape, and the ink is ejected. In this manner, a high displacement can be provided to the diaphragm  136  and a high ejection pressure can be accomplished. Moreover, a sufficient displacement can be obtained without thinning the film of the diaphragm  136  more than necessary or without forming the piezoelectric body  140  only in the area other than the region corresponding to the central section of the pressure chamber  112 , thus the rigidity can be secured. 
       FIG. 6  is a diagram showing an example of a drive waveform of the driving voltage applied to a piezoelectric actuator  134 . 
     Because the polarization direction of the film of the piezoelectric body (PZT)  140  that is formed by means of sputtering is opposite of the direction in which the piezoelectric body  140  is normally used, the potential of the lower electrode  142  serving as the common electrode is taken as 0 (V), and a negative potential (−V (V)) is applied to the upper electrode  144  serving as the individual electrode. According to the driving method used in this case, a voltage is applied in order to draw the ink, and the voltage is opened in order to eject the ink. By applying the driving voltage only when ejecting the ink, as described above, highly reliable drive of the inkjet head  100  can be achieved. 
     On the other hand, when the upper electrode  144  is formed into a circle, it is difficult to increase the generated pressure more than in the normal use because the voltage is opened to 0 (V) at the time of the ink ejection and therefore displacement is not applied forcibly. However, in the inkjet head  100  of the present embodiment described above, the piezoelectric actuator  134  is previously bent into a projecting shape protruding toward the pressure chamber, so that a high displacement can be secured and high ejection pressure can be obtained. 
     Note that the piezoelectric actuator  134  is configured to pass beyond the neutral point and to be displaced to the side opposite of the pressure chamber  112  side when applied with the driving voltage. Therefore, when the displacement with respect to the applied voltage is δL, the relationship thereof to an initial deflection amount δ0 (the amount of displacement from the neutral point during the initial state) is δL&gt;δ0. The thickness of the diaphragm  136 , the thickness of the piezoelectric body  140 , and the applied voltage need to be designed in order to obtain a desired deflection amount δ0 and displacement amount δL, for this inequality. 
     The parameters for controlling the initial deflection amount include the coefficient of linear expansion of the piezoelectric body  140  (the coefficient of linear expansion can be increased by changing the additive amount of Nb (niobium)), the material (the coefficient of linear expansion) and thickness of the lower electrode  142 . The piezoelectric actuator  134  is designed by combining these parameters. 
     Note in the embodiment described above that the diaphragm  136  is made of Si but the composition of the diaphragm  136  is not limited thereto. For example, metallic materials such as stainless steel, nickel, and aluminum can be used for forming the diaphragm  136 . 
     On the other hand, as in the inkjet head  100  of the present embodiment explained above, the piezoelectric actuator  134  that is bent into a projecting shape protruding toward the pressure chamber can be formed easily by forming the diaphragm  136  with Si having a low coefficient of linear expansion and forming the piezoelectric body  140  by means of a thin film forming method involving a heat treatment. 
     Similarly, the flow path substrate  132  can be configured using a metallic material such as stainless steel, in place of Si or other silicon materials. In addition, the nozzle plate  130  can also be formed using a resin material such as polyimide or a metallic material such as stainless steel, in place of Si or other silicon materials. 
     As in the inkjet head  100  of the present embodiment described above, the flow paths and the like can be formed accurately and the density of the nozzles can be increased, by forming the nozzle plate  130 , the flow path substrate  132 , and the diaphragm  136  by using Si. 
     In the present embodiment, although the lower electrode  142  is formed over the entire upper surface of the diaphragm  136 , the lower electrode  142  may be formed into an oval in accordance with the shape of the lower surface of the piezoelectric body  140  (oval shape). 
     Moreover, in the present embodiment, an example in which the present invention is applied to an inkjet head is described, but the application of the present invention is not limited to this embodiment. The present invention can be applied to a variety of liquid ejection heads that eject a conductive paste to form fine wiring patterns on a substrate, eject an organic light emitting material onto a substrate to form a high-definition display, or eject optical resin onto a substrate to form a minute electronic device such as an optical waveguide. 
     The shape of an individual electrode is not limited to a circular shape, and may be another shape such as a polygonal shape. 
     It should be understood that there is no intention to limit the invention to the specific forms disclosed, but on the contrary, the invention is to cover all modifications, alternate constructions and equivalents falling within the spirit and scope of the invention as expressed in the appended claims.