Patent Publication Number: US-2020290357-A1

Title: Ink jet head and ink jet printer

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is based upon and claims the benefit of priority from Japanese Patent Application No. 2019-045827, filed on Mar. 13, 2019, the entire contents of which are incorporated herein by reference. 
     FIELD 
     Embodiments described herein relate generally to an ink jet head and an ink jet printer. 
     BACKGROUND 
     An ink jet printer having a so-called shear mode type ink jet head structure in which ink droplets are ejected from nozzles by utilizing shear deformation of a piezoelectric member is known. In such a structure, for example, an insulating layer may be formed on electrodes to insulate the electrodes from ink having electrical conductivity or the like. 
     As an insulating material, for example, a film made of polyparaxylylene (Parylene® is known. When such an insulating material is formed by depositing polyparaxylylene after pretreating a surface of a support with a silane coupling agent, high adhesion can be obtained. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates a perspective view of an ink jet head according to an embodiment. 
         FIG. 2  illustrates an exploded perspective view of an actuator plate, a frame, and a nozzle plate included in the ink jet head according to the embodiment. 
         FIG. 3  illustrates a partially cut top view of the ink jet head according to the embodiment. 
         FIG. 4  illustrates a cross-sectional view along a plane perpendicular to a Y-axis in  FIG. 3 , illustrating a part of the ink jet head according to the embodiment. 
         FIG. 5  is a schematic diagram illustrating an ink jet printer according to an embodiment. 
         FIG. 6  is a graph illustrating an example of a relationship of a leakage current value of the electrode protective film to the number of times the voltage pulse is applied. 
         FIG. 7  is a graph illustrating another example of the relationship of the leakage current value of the electrode protective film to the number of times the voltage pulse is applied. 
     
    
    
     DETAILED DESCRIPTION 
     Example embodiments provide an ink jet head having excellent insulation durability and an ink jet printer equipped with such an ink jet head. 
     In general, according to an embodiment, an ink jet head includes a piezoelectric member, a plurality of electrodes, a protective lamination layer, and a nozzle plate. The piezoelectric member has a plurality of grooves. The electrodes are provided along surfaces of the grooves. The protective lamination layer covers the electrodes. The nozzle plate is provided on the piezoelectric member and having a plurality of ejection nozzles that faces the plurality of grooves. The protective lamination layer includes an insulating layer and a first oxide layer laminated on each other. The insulating layer contains an organic material. An oxygen content of the first oxide layer is greater than an oxygen content of the insulating layer. 
     According to another embodiment, an ink jet printer is provided. The ink jet printer includes an ink jet head according to an embodiment and a medium holding mechanism. The medium holding mechanism holds the recording medium facing the ink jet head. 
     1. Ink Jet Head 
     1-1. Configuration 
     Hereinafter, example embodiments will be described with reference to the drawings. 
       FIG. 1  illustrates a perspective view of an on-demand type ink jet head  1  to be mounted on a head carriage of an ink jet printer according to an embodiment. In the following description, an orthogonal coordinate system including an X-axis, a Y-axis, and a Z-axis is used. The X-axis direction corresponds to a print width direction. The Y-axis direction corresponds to a direction in which a recording medium is conveyed. The Z-axis direction is a direction facing towards a surface of the recording medium. 
     In  FIG. 1 , the ink jet head  1  includes an ink manifold  10 , an actuator plate  20 , a frame  40 , and a nozzle plate  50 . 
     The actuator plate  20  has a rectangular shape with a longitudinal direction along the X-axis direction. Examples of the material of the actuator plate  20  include alumina (Al 2 O 3 ), silicon nitride (Si 3 N 4 ), silicon carbide (SiC), aluminum nitride (AlN), and lead zirconate titanate (PZT: Pb(Zr,Ti)O 3 ). 
     The actuator plate  20  is overlaid on the ink manifold  10  so as to close an open end of the ink manifold  10 . The ink manifold  10  is connected to an ink cartridge via an ink supply pipe  11  and an ink return pipe  12 . 
     The frame  40  is attached on the actuator plate  20 . The nozzle plate  50  is attached on the frame  40 . A plurality of nozzles N is provided on the nozzle plate  50  at predetermined intervals along the X-axis direction so as to form two rows along the Y-axis. 
       FIG. 2  illustrates an exploded perspective view of the actuator plate  20 , the frame  40 , and the nozzle plate  50  included in the ink jet head according to the embodiment.  FIG. 3  illustrates a partially cut top view of the ink jet head according to the embodiment.  FIG. 4  illustrates a cross-sectional view along a plane perpendicular to the Y-axis in  FIG. 3 , illustrating a part of the ink jet head according to the embodiment. 
     This ink jet head  1  is a side-shooter type of a so-called shear mode shared-wall. 
     As illustrated in  FIGS. 2 and 3 , in the actuator plate  20 , a plurality of ink supply ports  21  are provided at intervals along the X-axis direction so as to form a row at a central portion in the Y-axis direction. In the actuator plate  20 , a plurality of ink discharge ports  22  are provided at intervals along the X-axis direction so as to respectively form rows in the plus Y-axis direction and the minus Y-axis direction with respect to the row of ink supply ports  21 . 
     A plurality of piezoelectric members  30  are provided between the row of ink supply ports  21  provided at the central portion and one row of ink discharge ports  22 . These piezoelectric members  30  form a row extending in the X-axis direction. The plurality of piezoelectric members  30  are also provided between the row of ink supply ports  21  provided at the central portion and the other row of ink discharge ports  22 . These piezoelectric members  30  also form a row extending in the X-axis direction. 
     As illustrated in  FIG. 4 , each of the rows of the plurality of piezoelectric members  30  includes a first piezoelectric body  301  and a second piezoelectric body  302  laminated on the actuator plate  20 . Examples of the material of the first piezoelectric body  301  and the second piezoelectric body  302  include lead zirconate titanate (PZT), lithium niobate (LiNbO 3 ), and lithium tantalate (LiTaO 3 ). The first piezoelectric body  301  and the second piezoelectric body  302  are polarized in opposite directions along the thickness direction. 
     In a laminate of the first piezoelectric body  301  and the second piezoelectric body  302 , a plurality of grooves each extending in the Y-axis direction and arranged in the X-axis direction are provided. These grooves are opened on the second piezoelectric body  302  side, and have a depth larger than the thickness of the second piezoelectric body  302 . Hereinafter, portions of the laminate that are sandwiched between adjacent grooves are referred to as channel walls. Each of these channel walls extends in the Y-axis direction and is arranged in the X-axis direction. 
     The piezoelectric member  30  forms a plurality of pressure chambers  32  at positions communicating with nozzles N, which are described below, and are configured to eject ink from the pressure chambers  32  by changing pressure in the pressure chambers  32 . Each pressure chamber  32  through which ink circulates is a space positioned in a groove between two adjacent channel walls. The width of the pressure chamber  32 , here, the dimension along the X-axis direction of the pressure chamber  32  is preferably in the range of 100 μm to 300 μm, and more preferably in the range of 20 μm to 60 μm. 
     An electrode  33  is formed on the side walls and the bottom of each of the pressure chambers  32 . That is, the electrode  33  is formed on a portion of the piezoelectric member  30  adjacent to the pressure chamber  32 . These electrodes  33  are connected to wiring patterns  31  extending along the Y-axis direction. The electrode  33  applies the drive pulse to the corresponding portion of the piezoelectric member  30 . 
     An electrode protective film  34  is formed on the surface of the actuator plate  20  including the electrode  33  and a wiring pattern  31  except for a connection portion at which a flexible printed board is connected. The electrode protective film  34  may be also referred to as an electrode protective lamination layer. The electrode protective film  34  will be described in detail below. 
     The frame  40  has an opening as illustrated in  FIGS. 2 and 3 . The opening is smaller than the actuator plate  20  and larger than a region of the actuator plate  20  where the ink supply port  21 , the piezoelectric member  30 , and the ink discharge port  22  are provided. The frame  40  is made of ceramics, for example. The frame  40  is joined to the actuator plate  20  by an adhesive, for example. 
     The nozzle plate  50  includes a nozzle plate substrate and a liquid repellent film provided on the medium facing surface (ejection surface for ejecting ink from the nozzles N). The nozzle plate substrate is made of, for example, a resin film such as a polyimide film. The liquid repellent film may be omitted. 
     The nozzle plate  50  is larger than the opening of the frame  40 . The nozzle plate  50  is joined to the frame  40  by an adhesive, for example. 
     In the nozzle plate  50 , a plurality of nozzles N that can eject ink toward the recording medium are provided. These nozzles N form two rows corresponding to the pressure chambers  32 . The nozzle N has a diameter that increases from the recording medium facing surface toward the pressure chamber  32 . The dimension of the nozzle N is set to a predetermined value according to an ink ejection amount. The nozzle N can be formed, for example, by performing laser machining using an excimer laser. 
     The actuator plate  20 , the frame  40 , and the nozzle plate  50  are integrated as illustrated in  FIG. 1 , and form a hollow structure. A region surrounded by the actuator plate  20 , the frame  40 , and the nozzle plate  50  is an ink circulation chamber. Ink is circulated in such a way that ink is supplied from the ink manifold  10  to the ink circulation chamber through the ink supply port  21 , passes through the pressure chamber  32 , and excess ink returns from the ink discharge port  22  to the ink manifold  10 . A part of the ink is ejected from the nozzle N while flowing through the pressure chamber  32  and is used for printing. 
     A flexible printed board  60  is connected to the wiring pattern  31  at a position outside the frame  40  on the actuator plate  20 . A drive circuit  61  that drives the piezoelectric member  30  is mounted on the flexible printed board  60 . 
     As illustrated in  FIG. 4 , the electrode protective film  34  includes a portion covering the electrode  33  and a portion of the surface of the second piezoelectric body  302  that covers a region  302   a  not covered by the electrode  33 . The latter portion can be omitted. 
     The electrode protective film  34  includes an insulating layer  34 A, a first oxide layer  34 B 1 , and a second oxide layer  34 B 2 . The electrode protective film  34  is a film having a three-layer structure in which the first oxide layer  34 B 1 , the insulating layer  34 A, and the second oxide layer  34 B 2  are laminated in this order in the thickness direction. 
     The insulating layer  34 A includes a portion facing the electrode  33  via the first oxide layer  34 B 1  and a portion facing the region  302   a . The latter portion can be omitted. 
     The insulating layer  34 A contains an organic substance. The insulating layer  34 A preferably has a higher withstand voltage than the first oxide layer  34 B 1  and the second oxide layer  34 B 2 . The insulating layer  34 A preferably has a lower moisture vapor transmission rate than the first oxide layer  34 B 1  and the second oxide layer  34 B 2 . 
     The organic substance preferably contains a compound having a polyparaxylylene backbone. According to an example, the insulating layer  34 A is made of the compound having a polyparaxylylene backbone. 
     The compound having the polyparaxylylene backbone preferably contains a repeating unit represented by the following general chemical formula (I): 
     
       
         
         
             
             
         
       
     
     In the general chemical formula (I), each of R1 to R8 independently represents a hydrogen atom or a halogen atom. Preferably, R1 to R4 are a hydrogen atom or a fluorine atom, and R5 to R8 are a hydrogen atom or a chlorine atom. 
     The compound having a polyparaxylylene backbone preferably comprises a compound in which, in the general chemical formula (I), all of R1 to R8 are hydrogen atoms or a compound in which R1 to R4 are hydrogen atoms, any one of R5 to R8 is a chlorine atom, and the other atoms of R5 to R8 are hydrogen atoms. That is, the insulating layer  34 A is preferably polyparaxylylene or polymonochloroparaxylylene. More preferably, the insulating layer  34 A comprises only polymonochloroparaxylylene. An example of the compound constituting the insulating layer  34 A includes diX® (manufactured by KISCO). 
     The thickness of the insulating layer  34 A is preferably between 1 μm and 15 μm, and more preferably between 5 μm and 10 μm. The thickness of the insulating layer  34 A can be measured, for example, by observing a cross-section in a scanning electron microscope (SEM). When the thickness of the insulating layer  34 A is increased, insulation durability of the ink jet head is improved. However, when the insulating layer  34 A is excessively thick, operation of the piezoelectric member  30  may be hindered. 
     The first oxide layer  34 B 1  includes a portion positioned between the electrode  33  and the insulating layer  34 A and a portion positioned between the region  302   a  and the insulating layer  34 A. The latter portion can be omitted in some examples. 
     The oxygen content of the first oxide layer  34 B 1  is greater than the oxygen content of the insulating layer  34 A. Here, the “oxygen content” represents an amount of oxygen per unit volume. Whether the oxygen content of the first oxide layer  34 B 1  is greater than the oxygen content of the insulating layer  34 A can be confirmed by X-ray photoelectron spectroscopy (XPS) analysis, for example. Specifically, the ink jet printer is disassembled and the electrode protective film  34  is collected. The XPS analysis is performed while etching the collected electrode protective film  34  in the thickness direction, thereby obtaining an O 1   s  spectrum. A difference in oxygen content between the oxide layer and the insulating layer can be confirmed from a plurality of O 1   s  spectra in the thickness direction. 
     The first oxide layer  34 B 1  includes, for example, an oxide of a metal or non-metallic element. The metal or non-metallic element is preferably at least one element selected from a group consisting of silicon (Si), titanium (Ti), aluminum (Al), hafnium (Hf), and tantalum (Ta). The oxide of the metal or non-metallic element preferably contains at least one oxide selected from a group consisting of SiO 2 , Al 2 O 3 , TiO 2 , HfO 2 , and Ta 2 O 5 . From the viewpoint of improving the adhesion with the electrode  33 , the oxide preferably contains at least one oxide selected from a group consisting of SiO 2 , Al 2 O 3 , and TiO 2 . 
     More preferably, the first oxide layer  34 B 1  includes SiO 2 . Since SiO 2  has a higher atomic ratio of oxygen than the other oxides exemplified above, SiO 2  is particularly good in increasing the insulation durability of the ink jet head. Since a dielectric constant of SiO 2  is small, parasitic capacitance can be reduced by using SiO 2 . Furthermore, when SiO 2  is used, a film having excellent flexibility can be obtained at low cost. 
     The thickness of the first oxide layer  34 B 1  is preferably 10 nm to 1000 nm. The thickness of the first oxide layer  34 B 1  is more preferably 100 nm to 500 nm. When the thickness of the insulating layer  34 A is increased, the insulation durability of the ink jet head is improved. However, when the insulating layer  34 A is excessively thickened, the operation of the piezoelectric member  30  may be hindered. The thickness of the first oxide layer  34 B 1  can be measured, for example, by observing a cross-section with the SEM. 
     The first oxide layer  34 B 1  may be an oxide film made of an oxide of the material of the insulating layer  34 A. The thickness of the first oxide layer  34 B 1  is preferably in the range of 10 nm or to 100 nm, and more preferably in the range of 20 nm to 60 nm. When the thickness of the surface region is too large, early insulation of the electrode protective film may be insufficient. When the thickness of the surface region is too small, long-term insulation may be insufficient. 
     The second oxide layer  34 B 2  covers the insulating layer  34 A. The second oxide layer  34 B 2  includes a portion positioned between the insulating layer  34 A and the pressure chamber  32  and a portion positioned between the insulating layer  34 A and the nozzle plate  50 . The latter portion can be omitted. 
     The second oxide layer  34 B 2  includes, for example, an oxide of a metal or non-metallic element. As the metal or non-metallic element and the oxide thereof, the same element and oxide as those described for the first oxide layer  34 B 1  can be used. From the viewpoint of excellent ink resistance, the second oxide layer  34 B 2  preferably contains at least one oxide selected from a group consisting of HfO 2  and Ta 2 O 5 . 
     The thickness of the second oxide layer  34 B 2  is preferably within the range described above for the first oxide layer  34 B 1 . 
     The second oxide layer  34 B 2  may be an oxide film formed by oxidizing the surface region of the insulating layer  34 A. That is, the second oxide layer  34 B 2  may be an oxide film made of an oxide of the material of the insulating layer  34 A. The film thickness of the second oxide layer  34 B 2  is preferably within the range described above for the first oxide layer  34 B 1  made of the oxide of the material of the insulating layer  34 A. 
     One of the first oxide layer  34 B 1  and the second oxide layer  34 B 2  may be omitted. 
     1-2. Ink Ejection 
     Hereinafter, the operation of the piezoelectric member  30  will be described with reference to  FIGS. 3 and 4 . Here, the operation will be described assuming that the pressure chambers  32  are also formed on both sides of the central pressure chamber  32 . It is assumed that the electrodes  33  corresponding to the three adjacent pressure chambers  32  are electrodes A, B and C, respectively, and the electrode  33  corresponding to the central pressure chamber  32  is the electrode B. 
     In order to eject ink from the nozzle N, first, for example, a voltage pulse having higher potential than potentials of the adjacent electrodes A and C is applied to the central electrode B to generate an electric field in a direction perpendicular to the channel wall. Thus, the channel walls are driven in the shear mode and a pair of channel walls sandwiching the central pressure chamber  32  is deformed so that the central pressure chamber  32  expands. 
     Next, a voltage pulse having higher potential than the potential of the central electrode B is applied to both adjacent electrodes A and C to generate an electric field in a direction perpendicular to the channel wall. Thus, the channel walls are driven in the shear mode and the pair of channel walls sandwiching the central pressure chamber  32  is deformed so that the central pressure chamber  32  is reduced. By this operation, pressure is applied to ink in the central pressure chamber  32  and the ink is ejected from the nozzle N corresponding to the pressure chamber  32  to land on the recording medium. Thus, in the ink jet head  1 , ink is ejected from the nozzle N using the piezoelectric member  30  as an actuator. 
     In the printing process using the ink jet head  1 , for example, all the nozzles N are divided into three groups and the driving operation described above is performed in a time-sharing manner for three cycles to perform printing on the recording medium. 
     1-3. Manufacturing Method 
     Next, a method for manufacturing the ink jet head  1  illustrated in  FIGS. 1 to 4  will be described. 
     The ink jet head  1  is manufactured by the following method. First, a structure including the piezoelectric member  30  and the electrode  33  is formed. Specifically, a structure including the piezoelectric member  30  that forms the pressure chamber  32  to which ink is supplied and ejects ink in the pressure chamber  32  by changing the pressure in the pressure chamber  32 , and the electrode  33  that is positioned in a portion of the piezoelectric member  30  adjacent to the pressure chamber  32  and applies the drive pulse to the piezoelectric member  30  is formed. The structure can be formed by a method known in the related art. 
     Next, the electrode protective film  34  is formed on the electrode  33  and the region  302   a  by a method described below. Thereafter, the nozzle plate  50  is installed so that the nozzle N communicates with the pressure chamber  32 . 
     An example of a method for manufacturing the electrode protective film  34  will be described. 
     First, the first oxide layer  34 B 1  is formed on the electrode  33  and the region  302   a . For example, first, dispersion liquid in which transition element oxide particles are dispersed in a dispersion medium is prepared. As the dispersion medium, water or an organic solvent may be used. The dispersion liquid may further contain a binding agent. Next, this dispersion liquid is coated onto the electrode  33  and the region  302   a  by using, for example, a spin coat method, a spray method, or the like to form a coating film. This coating film is dried to obtain the first oxide layer  34 B 1 . The first oxide layer  34 B 1  may be formed by a sol-gel method. 
     The first oxide layer  34 B 1  may be formed by a chemical vapor deposition (CVD) method. When the SiO 2  film is used as the first oxide layer  34 B 1 , it is preferable to use a plasma-enhanced chemical vapor deposition (PECVD) method using tetraethyl orthosilicate (TEOS) as a raw material. When the PECVD method is used, production efficiency can be increased. 
     Next, the insulating layer  34 A is formed on the first oxide layer  34 B 1 . Specifically, first, an organic substance is prepared. As the organic substance, for example, a compound having a polyparaxylylene skeleton can be used. The organic substance is deposited on the first oxide layer  34 B 1  by using a method known in the related art such as a vapor deposition method to form the insulating layer  34 A. 
     Next, the second oxide layer  34 B 2  is formed on the insulating layer  34 A. For example, the second oxide layer  34 B 2  is obtained by oxidizing the surface of the insulating layer  34 A. Examples of the method for oxidizing the surface of the insulating layer  34 A include ultraviolet irradiation treatment, plasma treatment, and ozone treatment in an oxygen-containing atmosphere. From the viewpoint that the insulating layer  34 A is hardly deteriorated, it is preferable to use an ultraviolet irradiation treatment as a surface treatment method of the insulating layer  34 A. In the ultraviolet irradiation, the illuminance is preferably 10 mW/cm 2  to 20 mW/cm 2 , and the irradiation time is 3 to 10 minutes. 
     Alternatively, the second oxide layer  34 B 2  may be formed by a method similar to that of the first oxide layer  34 B 1 . 
     Next, another example of the method for manufacturing the electrode protective film  34  will be described. 
     First, a layer made of the same material as the insulating layer  34 A is formed on a base material. For formation of this layer, the same method as described above for the insulating layer  34 A can be used. Next, this layer is oxidized to obtain the first oxide layer  34 B 1 . 
     Next, the insulating layer  34 A is formed on the first oxide layer  34 B 1 . The insulating layer  34 A can be formed by the same method as described above. Thereafter, the surface region of the insulating layer  34 A is oxidized to obtain the second oxide layer  34 B 2 . 
     Although the method for forming the electrode protective film  34  having a three-layer structure is described here, the electrode protective film  34  having a two-layer structure may be formed by omitting the formation of the first oxide layer  34 B 1  or the second oxide layer  34 B 2 . 
     2. Ink Jet Printer 
     2.1 Configuration 
       FIG. 5  illustrates a schematic diagram of an ink jet printer  100 . 
     The ink jet printer  100  according to the embodiment includes ink jet heads  115 C,  115 M,  115 Y, and  115 Bk, and a medium holding mechanism  110  that holds the recording medium facing the ink jet heads  115 C,  115 M,  115 Y, and  115 Bk. Each of the ink jet heads  115 C,  115 M,  115 Y, and  115 Bk is the ink jet head  1  described with reference to  FIGS. 1 and 2 . 
     The ink jet printer  100  illustrated in  FIG. 5  includes a casing including a paper discharge tray  118 . In the casing, cassettes  101   a  and  101   b , paper feed rollers  102  and  103 , conveyance roller pairs  104  and  105 , a registration roller pair  106 , a conveyance belt  107 , a fan  119 , a negative pressure chamber  111 , conveyance roller pairs  112 ,  113  and  114 , ink jet heads  115 C,  115 M,  115 Y, and  115 Bk, ink cartridges  116 C,  116 M,  116 Y, and  116 Bk, and tubes  117 C,  117 M,  117 Y, and  117 Bk are installed. 
     The cassettes  101   a  and  101   b  accommodate recording media P of different sizes. The paper feed roller  102  or  103  picks up the recording medium P corresponding to the size of a selected recording medium from the cassette  101   a  or  101   b  and conveys the recording medium P to the conveyance roller pairs  104  and  105  and the registration roller pair  106 . 
     The conveyance belt  107  is tensioned by a driving roller  108  and two driven rollers  109 . Holes are provided on the surface of the conveyance belt  107  at predetermined intervals. The negative pressure chamber  111  connected to the fan  119  for holding the recording medium P to the conveyance belt  107  is installed inside the conveyance belt  107 . The conveyance roller pairs  112 ,  113 , and  114  are installed downstream of the conveyance belt  107  in the conveyance direction. A heater for heating a printed layer formed on the recording medium P can be installed in a conveyance path from the conveyance belt  107  to the paper discharge tray  118 . 
     Above the conveyance belt  107 , four ink jet heads that eject ink onto the recording medium P according to image data are disposed. Specifically, an ink jet head  115 C that ejects cyan (C) ink, an ink jet head  115 M that ejects magenta (M) ink, an ink jet head  115 Y that ejects yellow (Y) ink, and an ink jet head  115 Bk that ejects black (Bk) ink are disposed in this order from the upstream side. Each of the ink jet heads  115 C,  115 M,  115 Y, and  115 Bk is the ink jet head  1  described with reference to  FIGS. 1 and 2 . 
     Above the ink jet heads  115 C,  115 M,  115 Y, and  115 Bk, a cyan (C) ink cartridge  116 C, a magenta (M) ink cartridge  116 M, a yellow (Y) ink cartridge  116 Y, and a black (Bk) ink cartridge  116 Bk that respectively contain inks corresponding to the ink jet heads  115 C,  115 M,  115 Y, and  115 Bk are installed. These ink cartridges  116 C,  116 M,  116 Y, and  116 Bk are connected to the ink jet heads  115 C,  115 M,  115 Y, and  115 Bk by the tubes  117 C,  117 M,  117 Y, and  117 Bk, respectively. 
     2-2. Image Formation 
     Next, an image forming operation of the ink jet printer  100  will be described. 
     First, an image processing unit starts image processing, generates an image signal corresponding to image data, and generates a control signal for controlling operations of various rollers, the negative pressure chamber  111 , and the like. 
     Under the control of the image processing unit, the paper feed roller  102  or  103  picks up the recording medium P of the selected size from the cassette  101   a  or  101   b  and conveys the recording medium P to the conveyance roller pair  104  or  105  and the registration roller pair  106 . The registration roller pair  106  corrects skew of the recording medium P and conveys the recording medium P at a predetermined timing. 
     The negative pressure chamber  111  suctions air through the holes of the conveyance belt  107 . Accordingly, the recording medium P is sequentially conveyed to positions below the ink jet heads  115 C,  115 M,  115 Y, and  115 Bk as the conveyance belt  107  moves in a state of being attracted to the conveyance belt  107 . 
     The ink jet heads  115 C,  115 M,  115 Y, and  115 Bk eject ink in synchronization with the timing at which the recording medium P is conveyed under the control of the image processing unit. With this configuration, a color image is formed at a desired position on the recording medium P. 
     Thereafter, the conveyance roller pairs  112 ,  113 , and  114  discharge the recording medium P on which the image is formed to the paper discharge tray  118 . When a heater is installed in the conveyance path from the conveyance belt  107  to the paper discharge tray  118 , the print layer formed on the recording medium P may be heated by the heater. When heating with the heater is performed, particularly when the recording medium P is impermeable, adhesion of the print layer to the recording medium P can be improved. 
     3. Effect 
     The ink jet head  1  described above includes the electrode protective film  34  in which the first oxide layer  34 B 1  and the second oxide layer  34 B 2  each having a higher oxygen content than that of the insulating layer  34 A are laminated on both surfaces of the insulating layer  34 A. According to such a configuration, excellent insulation durability can be achieved. The reason will be described below. 
     One of the possible reasons why an ink jet head that does not include the first oxide layer  34 B 1  and the second oxide layer  34 B 2 , may not achieve insulation durability is that organic molecules on the electrode are destroyed in the insulating layer covering the electrode. Such destruction is considered to be caused by the following reason. 
     In the shear mode type ink jet head, an AC voltage is applied to the piezoelectric member. Accordingly, an AC voltage is also applied to an electrode used for applying a voltage to the piezoelectric element, and inks are adjacent to the electrode with the insulating layer interposed therebetween. That is, both the electrode and the ink can be an anode or a cathode. 
     In this case, in the region of the insulating layer in contact with the anode, electrons are separated from the organic molecules contained in the insulating layer, and the separated electrons may move to the anode. In the portion where the electrons are separated in the insulating layer, vacancies, that is, holes formed by escape of electrons, are formed (hole injection). When such a movement of electrons is repeated and then ultimately exceeds a certain amount, destruction of the organic molecules constituting the insulating layer occur in a region near the anode. As a result, it is considered that dielectric breakdown occurs in the insulating layer. 
     In the ink jet head  1  according to the embodiment, the first oxide layer  34 B 1  and the second oxide layer  34 B 2  having a higher oxygen content than that of the insulating layer  34 A are disposed between the electrode  33  and the insulating layer  34 A and between the ink and the insulating layer  34 A, respectively. That is, in the ink jet head  1 , the first oxide layer  34 B 1  or the second oxide layer  34 B 2  is interposed between the insulating layer  34 A and the anode. Oxygen has a relatively large electronegativity. For that reason, the first oxide layer  34 B 1  and the second oxide layer  34 B 2  more easily donate electrons to the anode than the insulating layer  34 A. Accordingly, when the first oxide layer  34 B 1  and second oxide layer  34 B 2  are in contact with the anode, hole injection into the insulating layer  34 A can be suppressed. Therefore, it is more difficult to cause dielectric breakdown due to deterioration of the insulating layer  34 A. 
     As described above, both the electrode  33  and the ink can be an anode and a cathode. Accordingly, as described above, it is preferable to dispose the first oxide layer  34 B 1  and the second oxide layer  34 B 2  between the electrode  33  and the insulating layer  34 A and between the ink and the insulating layer  34 A, respectively. However, even when one of the first oxide layer  34 B 1  and the second oxide layer  34 B 2  is omitted, it is still possible to make it somewhat more difficult to cause dielectric breakdown due to deterioration of the insulating layer  34 A as compared to the case where both the first oxide layer  34 B 1  and the second oxide layer  34 B 2  are omitted. 
     Either one of the first oxide layer  34 B 1  and the second oxide layer  34 B 2  may be omitted in some examples. However, it is generally preferable not to omit the first oxide layer  34 B 1 . This is because movement of electrons to the electrode  33  is easier to occur than movement of electrons to the ink 
     EXAMPLES 
     Ink Jet Head Manufacturing 
     Example 1 
     The ink jet head  1  illustrated in  FIGS. 1 to 4  was manufactured as follows. 
     First, a structure including the piezoelectric member  30  and the electrode  33  was formed. Next, the insulating layer  34 A and the second oxide layer  34 B 2  were laminated on the electrode  33  in this order. 
     Specifically, a film made of polyparaxylylene (Parylene® C) was formed on the electrode  33  by a vapor deposition method to obtain the insulating layer  34 A. The thickness of the insulating layer  34 A was 5 μm. 
     Next, an ethanol solution containing the TEOS was coated onto the insulating layer  34 A by a spin coating method to form a coating film. This coating film was dried at room temperature to obtain a SiO 2  film as the second oxide layer  34 B 2 . The film thickness of the second oxide layer  34 B 2  was 0.5 μm. 
     Subsequently, the nozzle plate  50  was installed so that the nozzle N communicated with the pressure chamber  32 , and the ink jet head  1  was obtained. 
     Example 2 
     Instead of forming the SiO 2  film as the second oxide layer  34 B 2 , the surface region of the insulating layer  34 A was irradiated with ultraviolet rays to form an oxide film. Other than this, the ink jet head  1  was obtained in the same manner as in Example 1. In the ultraviolet irradiation, the illuminance was 17 mW/cm 2  and the irradiation time was 5 minutes. The film thickness of the second oxide layer  34 B 2  was 30 nm. 
     Example 3 
     The ink jet head  1  was obtained in the same manner as in Example 1 except that the first oxide layer  34 B 1  was provided prior to formation of the insulating layer  34 A. The first oxide layer  34 B 1  was formed on the electrode  33  in the same manner as the second oxide layer  34 B 2  of Example 1. The film thickness of the first oxide layer  34 B 1  was 1 μm. 
     Comparative Example 1 
     An ink jet head was obtained in the same manner as described in Example 1 except that the formation of the second oxide layer  34 B 2  was omitted. 
     Evaluation 
     A voltage was applied to the electrode  33  of the ink jet head obtained in the Examples and Comparative Example, and a leakage current value was observed. Specifically, first, a voltage pulse having an amplitude of 60 V was applied to the electrode  33  of the ink jet head 1×10 8  times. Thereafter, current leakage between the electrode  33  and the ink was measured. The same measurement was performed on the ink jet head to which the voltage pulse was applied 1×10 9  times, 1×10 10  times, 1×10 11  times, and 1×10 12  times. 
     The results of Examples 1 and 2 and Comparative Example 1 are illustrated in  FIG. 6 .  FIG. 6  is a graph illustrating an example of the relationship of a leakage current value of the electrode protective film to the number of times the voltage pulse is applied. 
     As illustrated in  FIG. 6 , in the ink jet head according to Comparative Example 1, the leak current value was significantly increased by applying the voltage 1×10 11  times or more. On the other hand, in the ink jet heads according to Examples 1 and 2, the leakage current value did not change even when the voltage application was repeated 1×10 11  times. That is, excellent insulation durability was achieved in Examples 1 and 2. When the SiO 2  film was used as the second oxide layer  34 B 2 , a fact that the leakage current value was smaller and the insulation durability was more excellent was exhibited than when an UV treatment film of the parylene film was used as the second oxide layer  34 B 2 . 
     The results of Example 3 and Comparative Example 1 are illustrated in  FIG. 7 .  FIG. 7  is a graph illustrating another example of the relationship of the leakage current value of the electrode protective film to the number of times the voltage pulse is applied. 
     As illustrated in  FIG. 7 , in the ink jet head according to Example 3, the leak current value did not change even when the voltage application was repeated 1×10 12  times. That is, excellent insulation durability was achieved in Example 3. The insulation durability of the inkjet head according to Example 3 using the electrode protective film having the three-layer structure of the first oxide layer, the insulating layer, and the second oxide layer was superior to the insulation durability of the ink jet heads according to Examples 1 and 2 using the electrode protective film having a two-layer structure of the insulating layer and the second oxide layer. 
     An ink jet head according to example embodiments described above includes an insulating protective film in which an insulating layer and an oxide layer having a higher oxygen content than that of the insulating layer are laminated on each other. Accordingly, an ink jet head according to an example embodiment can maintain insulation for a long period of time. 
     While certain embodiments have been described, these embodiments have been presented by way of example only, and are not intended to limit the scope of the inventions. Indeed, the novel embodiments described herein may be embodied in a variety of other forms; furthermore, various omissions, substitutions and changes in the form of the embodiments described herein may be made without departing from the spirit of the inventions. The accompanying claims and their equivalents are intended to cover such forms or modifications as would fall within the scope and spirit of the inventions.