Patent Publication Number: US-2011058852-A1

Title: Charging device and electrophotographic image forming apparatus including the same

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority under 35 U.S.C. §119 from Korean Patent Application No. 10-2009-0084435, filed on Sep. 8, 2009, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference. 
     BACKGROUND 
     1. Field of the Invention 
     The present general inventive concept relates to a charging device and an electrophotographic image forming apparatus including the same, and more particularly, to a charging device using a nano structure and an electrophotographic image forming apparatus including the same. 
     2. Description of the Related Art 
     Electrophotographic image forming apparatuses charge a photoreceptor such as a photosensitive drum uniformly, scan a laser beam on the charged photoreceptor to form an electrostatic latent image, make the electrostatic latent image visible by using a toner, thereby forming an image. Electrophotographic image forming apparatuses are used in a digital printer or a digital copying machine. 
     A charging device that charges such a photoreceptor operates by using a contact method by which a charging member contacts the photoreceptor, or by using a non-contact method by which the charging member does not contact the photoreceptor by using a corona discharge. In a charging device using the contact method using a charging member such as a charging roller, the life-span of the photoreceptor may be reduced due to the contact between the charging member and the photoreceptor. Thus, a non-contact charging device is widely used as an image forming apparatus that requires a long life-span. 
     The non-contact charging device charges the photoreceptor by using a corona discharge by using a charging wire or a charging pin. The non-contact charging device generates a discharge product such as ozone or a nitrogen oxide during a charging operation. The discharge product is harmful to the human body and thus, the amount of discharge product generated has to be reduced. 
     SUMMARY 
     The present general inventive concept provides a charging device to more uniformly charge a surface of a photoreceptor and to reduce an amount of a discharge product, such as ozone or a nitrogen oxide, generated therefrom, and an electrophotographic image forming apparatus including the same. 
     Additional aspects and utilities of the present general inventive concept will be set forth in part in the description which follows and, in part, will be obvious from the description, or may be learned by practice of the general inventive concept. 
     The foregoing and/or other features and utilities of the present general inventive concept may be achieved by providing a charging device including a substrate formed of a conductive material; a porous insulating layer disposed on the substrate and including a plurality of hollow rods vertically formed on the substrate, a nano structure formed in the plurality of hollow rods, having conductivity and electrically and mechanically connected to the substrate, a grid electrode separated from the nano structure, and a spacer to support both ends of the grid electrode. 
     The substrate may include metal. 
     A horizontal cross-section of the substrate that is parallel to a surface on which the porous insulating layer is formed may have a rectangular shape in which a length corresponding to a width of an object to be charged in a first direction is relatively long and a width in a second direction that is perpendicular to the first direction is relatively short. 
     A length of the substrate having the rectangular shape in the first direction may be uniform regardless of a third direction that is perpendicular to the first and second directions and a width of the substrate having the rectangular shape in the second direction may be decreased as it gets closer to a top surface of the substrate in the third direction or may be uniform. 
     A length of the substrate in the third direction may be equal to or greater than a maximum width of the substrate in the second direction. 
     The substrate may include a plurality of protrusions protruding from the top surface of the substrate, and the porous insulating layer may be formed on a top surface of each of the plurality of protrusions. 
     The plurality of protrusions may be arranged in a row in a dotted line at equal intervals. 
     The plurality of protrusions may be arranged in a plurality of rows in a dotted line at equal intervals and may be alternately arranged between the plurality of neighboring rows. 
     When the number of rows of the plurality of protrusions is N and a pitch interval between the protrusions in each of the rows is P, the protrusions between the neighboring rows may be alternately arranged at a difference of P/N. 
     Each of the plurality of protrusions may have a shape in which an area of a surface on which the porous insulating layer is to be formed is equal to or smaller than an area of a surface on which the substrate is to be formed. 
     The porous insulating layer may be a nanoporous template. 
     The nanoporous template may be an anodizing alumina template or a polymer nano template. 
     The nano structure may be a metal nano rod that fills the hollow rods of the porous insulating layer or carbon nanotubes that are grown in the hollow rods of the porous insulating layer. 
     The nano structure may fill the hollow rods of the porous insulating layer or portions thereof. 
     The nano structure may be in the shape of a continuous strip on the substrate. 
     An area of the grid electrode that corresponds to the continuous line shape of the nano structure may be opened. 
     A group of nano structures may constitute non-continuous islands, and the islands may be arranged in a row on the substrate in a dotted line at equal intervals. 
     The grid electrode may be connected to the nano structure while passing an area in which the dotted line shape of the nano structure is projected, or while passing a reverse image of the area in which the dotted line shape of the nano structure is projected. 
     A group of nano structures may constitute non-continuous islands, and the islands may be arranged in a plurality of rows on the substrate in a dotted line at equal intervals, and the islands may be alternately arranged between the neighboring rows. 
     The spacer may be interposed between the grid electrode and the substrate or between the grid electrode and the porous insulating layer. 
     The spacer may include an insulating material. 
     The device may further include at least one grid holder disposed between both ends of the grid electrode and supporting the grid electrode while being inserted or fixed in the substrate. 
     The device may further include a voltage source applying a charging voltage to the substrate and applying an adjustment voltage to the grid electrode, wherein an absolute value of the adjustment voltage is smaller than an absolute value of the charging voltage. 
     The foregoing and/or other features and utilities of the present general inventive concept may also be achieved by providing a charging device that charges an object to be charged, the device including a substrate formed of a conductive material, and a plurality of protrusions two-dimensionally arranged on the substrate and having conductivity. 
     The device may further include: a porous insulating layer disposed on a top surface of each of the plurality of protrusions and including a plurality of hollow rods vertically formed on the top surface of each of the protrusions, and a nano structure formed in the plurality of hollow rods, having conductivity and electrically and mechanically connected to the substrate. 
     The plurality of protrusions may be arranged in a plurality of rows in a dotted line at equal intervals and may be alternately arranged between the plurality of neighboring rows. 
     When the number of rows of the plurality of protrusions is N and a pitch interval between the protrusions in each of the rows is P, the protrusions between the neighboring rows may be alternately arranged at a difference of P/N. 
     Each of the plurality of protrusions may have a shape in which an area of a surface on which the porous insulating layer is to be formed is equal to or smaller than an area of a surface on which the substrate is to be formed. 
     The substrate may include metal. 
     The porous insulating layer may be an anodizing alumina template or a polymer nano template. 
     The nano structure may be a metal nano rod that fills the hollow rods of the porous insulating layer or carbon nanotubes that are grown in the hollow rods of the porous insulating layer. 
     The foregoing and/or other features and utilities of the present general inventive concept may also be achieved by providing an electrophotographic image forming apparatus including a photoreceptor, a charging device described above and later to charge an outer surface of the photoreceptor, a light scanning unit to scan light onto the outer surface of the photoreceptor to form an electrostatic latent image, and a developing unit to supply a toner to the electrostatic latent image formed on the photoreceptor to develop a toner image. 
     The foregoing and/or other features and utilities of the present general inventive concept may also be achieved by providing a charging device usable in an image forming apparatus to charge an object, the charging device including a substrate, and a charging electrode formed on the substrate, disposed toward the object, and having tip portions spaced apart from each other to charge the object. 
     The tip portions of the charging electrode may correspond to a width of a charging area of the object. 
     The charging device may further include a grid electrode disposed between the object and the tip portions of the charging electrode to control charging uniformity. 
     The charging device may further include a grid holder to hold and support the grid electrode with respect to the charging electrode. 
     The charging electrode may include a plurality of nano structures on which the respective tip portions are formed, and the nano structures comprises a first portion formed on the substrate and a second portion extended from the first portion and having an area narrower than the first portion according to a distance from the substrate. 
     The charging electrode may include a plurality of nano structures on which the respective tip portions are formed, and the nano structures are disposed in a direction perpendicular to a surface of the substrate. 
     The charging electrode may include a plurality of nano structures on which the respective tip portions are formed, and the plurality of nano structures may be disposed on an area of the substrate, and the area of the substrate may corresponds to a charging area of the object. 
     The charging electrode may include a plurality of nano structures on which the respective tip portions are formed, and the plurality of nano structures may include a first group of nano structures disposed in a direction and spaced-apart from each other by an interval, and a second group of nano structures disposed in a second direction within the interval. 
     The charging electrode may include a plurality of nano structures on which the respective tip portions are formed, and the tip portions may include a first group of tip portions disposed in a direction and spaced-apart from each other by an interval, and a second group of tip portions disposed in a second direction within the interval. 
     The second tip portions may be spaced apart from each other by another interval. 
     The second group of tip portions may be disposed in an area defined by the adjacent ones of the first group of tip portions and the interval with respect to the second direction. 
     The object may rotate in a rotation direction and with respect to a rotation axis, and the second direction is disposed between the rotation direction and the rotation axis to increase density of charging characteristic with the interval. 
     The first direction may be parallel to the rotation axis, and the second direction may not be parallel to the rotation axis and the first direction. 
     The charging electrode may include an insulation layer, at least two rods formed in the insulation layer and formed with the corresponding tip portions, and nano structures formed in corresponding rods, the insulation layer has a height, and the nano structures have second heights. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other features and advantages of the present general inventive concept will become more apparent by describing in detail exemplary embodiments thereof with reference to the attached drawings in which: 
         FIG. 1  is a schematic cross-sectional view of a charging device according to an embodiment of the present general inventive concept; 
         FIG. 2  is a perspective view of the charging device of  FIG. 1 ; 
         FIG. 3  is a perspective view of a charging electrode disposed in the charging device of  FIG. 2 ; 
         FIG. 4  is an enlarged view of a region A of  FIG. 3 ; 
         FIG. 5  illustrates a charging electrode that extends in a continuous line along a lengthwise direction of a substrate, according to an embodiment of the present general inventive concept; 
         FIG. 6  is an enlarged view of a region B of  FIG. 5 ; 
         FIG. 7  is a partial perspective view of the region B of  FIG. 5 ; 
         FIG. 8  is a partial perspective view of a nano structure of a tip portion of the region B of  FIG. 5 , according to an embodiment of the present general inventive concept; 
         FIGS. 9A and 9B  illustrate a tip portion of the region B of  FIG. 5 , according to other embodiments of the present general inventive concept; 
         FIG. 10  is an enlarged view of a region C of  FIG. 2 ; 
         FIG. 11  is a partial top view of the region C of  FIG. 2 ; 
         FIG. 12  is an enlarged view of a region D of  FIG. 2 ; 
         FIG. 13  is a side view of the region D of  FIG. 2 ; 
         FIG. 14  is a perspective view of a charging electrode disposed in a charging device according to another embodiment of the present general inventive concept; 
         FIG. 15  is a partial enlarged view of a region E of  FIG. 14 ; 
         FIG. 16  is a partial enlarged view of a region F of  FIG. 15 ; 
         FIG. 17  illustrates a protrusion according to an embodiment of the present general inventive concept; 
         FIG. 18  is a top view of the region E of  FIG. 14 ; 
         FIG. 19  is a side view of the region E of  FIG. 14 ; 
         FIG. 20  illustrates the distribution of electric fields between a charging electrode and an object to be charged; 
         FIG. 21  is a perspective view of a charging electrode disposed in a charging device according to another embodiment of the present general inventive concept; 
         FIG. 22  is a partial enlarged view of a region G of  FIG. 21 ; 
         FIG. 23  is a partial enlarged view of a region H of  FIG. 22 ; 
         FIG. 24  illustrates a protrusion according to an embodiment of the present general inventive concept; 
         FIG. 25  is a top view of the region G of  FIG. 21 ; 
         FIG. 26A  is a side view of the region G of  FIG. 21 , and  FIG. 26B  is a view illustrating arrangement of tip portions; and 
         FIG. 27  is a schematic view of an electrophotographic image forming apparatus including a charging device, according to an embodiment of the present general inventive concept. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     Reference will now be made in detail to the embodiments of the present general inventive concept, examples of which are illustrated in the accompanying drawings, wherein like reference numerals refer to the like elements throughout. The embodiments are described below in order to explain the present general inventive concept by referring to the figures. The invention may, however, be embodied in many different forms and should not be construed as being limited to the embodiments set forth herein; rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the concept of the present general inventive concept to those of ordinary skill in the art. In the drawings, the thicknesses of layers and regions are exaggerated for clarity. 
       FIGS. 1 through 13  illustrate a charging device according to an embodiment of the present general inventive concept. 
     In detail,  FIG. 1  is a schematic cross-sectional view of a charging device  100  according to an embodiment of the present general inventive concept, and  FIG. 2  is a perspective view of the charging device  100  of  FIG. 1 . For convenience of explanation, a grid holder is not shown in  FIG. 1 . 
     Referring to  FIGS. 1 and 2 , the charging device  100  according to the present embodiment charges an object  10  in a non-contact state and includes a charging electrode  110  and a grid electrode  190 . The charging electrode  110  includes a substrate  120 , and a nano structure  130  disposed on the substrate  120  and electrically connected to the substrate  120 . The nano structure  130  is formed and supported by a plurality of hollow rods  140   a  of a porous insulating layer  140 . In other words, a tip portion  150  of the charging electrode  110  includes the nano structure  130  and the porous insulating layer  140  that supports the nano structure  130 . A spacer  170  and a grid holder  180  are disposed between the charging electrode  110  and the grid electrode  190 . The spacer  170  and the grid holder  180  separate the grid electrode  190  from the charging electrode  110 . 
     A material  130   a  may be filled in the hollow rods  140   a  to define the nano structure  130 . The material  130   a  may be a conductive material and different from a material of the porous insulating layer  140 . The porous insulating layer  140  may have holes, and the material  130   a  may be filled in the holes to define the hollow rods  140   a . It is possible that the hollow rods  140   a  may be disposed in the holes of the porous insulating layer  140 , and the hollow rods  140  may be formed by the material  130   a  to have a diameter or a thickness to define a hollow space (or cylindrical shape space) therein. It is also possible that the hollow rods  140   a  may be disposed in the holes of the porous insulating layer  140 , and the material  130   a  may be filled in the hollow space (or cylindrical space) of the hollow rods  140 . In this case, the material  130   a  may be surrounded by the hollow rods  140   a  and the substrate  120 . 
     A charging voltage source that applies a charging voltage Ve is electrically connected to the charging electrode  110 , and an adjustment voltage source that applies an adjustment voltage Vg is electrically connected to the grid electrode  190 . An absolute value of the adjustment voltage Vg is smaller than an absolute value of the charging voltage Ve. The object  10  has a structure in which a layer to be charged  15  is formed at an outer circumferential surface of a support  11 . The object  10  may be a photoreceptor of an image forming apparatus. In this case, the object  10  is a photosensitive layer, and the support  11  is formed of a conductive metal such as aluminum (Al). The support  11  of the object  10  is grounded or is electrically connected to a voltage source that applies a lower voltage than the charging voltage Ve and the adjustment voltage Vg. 
     The object  10  may have a width W to correspond to an image which is formed thereon, developed, and transferred to a print medium. Since the forming, developing and transferring of the image are well known, detailed descriptions thereof will be omitted. The width W may correspond to a width of a print medium in a direction perpendicular to a feeding direction of the print medium, that is, an x direction. The object  10  is charged by the charging electrode  110 . Since the object  10  rotates about a rotating axis parallel to a first direction x, and the charging electrode  110  has a two dimension in a plane defined by the first direction x and a second direction y, a two dimension surface of the object  10  can be simultaneously and/or continuously charged by the charging electrode  110 . The two dimension of the charging electrode  110  may include a length in the first direction x to correspond to the width W of the object. 
       FIG. 3  is a perspective view of the charging electrode  110 . Referring to  FIG. 3 , the substrate  120  that constitutes a body of the charging electrode  110  is formed of a conductive material such as metal, for example, Al. 
     The substrate  120  has a rectangular shape in which a top surface ( 120   a  of  FIG. 4 ) of the substrate  120  is relatively long and a width thereof is relatively short. A length of the top surface  120   a  of the substrate  120  in the first direction x corresponds to a width of the layer to be charged  15  of the object  10 . Also, a width of the top surface  120   a  of the substrate  120  in the second direction y may be several tens of μm to several mm. The substrate  120  may have a horizontal cross-section that is parallel to the top surface  120   a  and is maintained to have a rectangular shape. In this case, a length of the horizontal cross-section that is parallel to the top surface  120   a  of the substrate  120  in the first direction x is uniform regardless of a third direction z. Also, the closer to the top surface  120   a  of the substrate  120 , the smaller a width of the horizontal cross-section that is parallel to the top surface  120   a  of the substrate  120  in the second direction y, or in other words, the width of the horizontal cross-section that is parallel to the top surface  120   a  of the substrate  120  in the second direction y may be tapered or narrowed as it gets closer to the top surface  120   a  in the third direction z. Meanwhile, a height of the substrate  120  in the third direction z may be the same as or greater than a maximum width of the cross-section that is parallel to the top surface  120   a  of the substrate  120 . Here, the first direction x, the second direction y, and the third direction z are perpendicular to each other. 
       FIG. 4  is a partial enlarged view of a region A of  FIG. 3 , and  FIG. 5  illustrates only the tip portion  150  of the charging electrode  110 .  FIG. 6  is an enlarged view of a region B of  FIG. 5 , and  FIG. 7  is a partial perspective view of the region B of  FIG. 5 . 
     Referring to  FIGS. 4 and 5 , the tip portion  150  that includes the nano structure  130  and the porous insulating layer  140  may be formed on the top surface  120   a  of the substrate  120  has a continuous strip shape. 
     The porous insulating layer  140  includes a plurality of hollow rods  140   a  that are formed in a direction perpendicular to the substrate  120 . The hollow rods  140   a  of the porous insulating layer  140  are two-dimensionally arranged at regular or irregular intervals when viewed from a top surface of the porous insulating layer  140 , as illustrated in  FIG. 6 . The porous insulating layer  140  may be formed of an insulating material such as an insulating metal oxide, polymer, ceramics, glass or an inorganic material. 
     The plurality of hollow rods  140   a  each having a nano size of several nm to several hundreds of nm may be formed on the porous insulating layer  140  by using various general methods. For example, a nanoporous template including a plurality of hollow rods each having a diameter of several nm to several hundreds of nm may be formed by using a self-assembling method of forming a self-aligned nano structure. The nanoporous template may be an anodizing alumina template or a polymer nano template. In addition, the porous insulating layer  140  may also be formed by using a photolithography technology or a nano imprinting technology. 
     When an anodizing alumina template is used to form the porous insulating layer  140 , all portions or portions of the top surface  120   a  of the substrate  120  formed of Al are oxidized so that the substrate  120  and the porous insulating layer  140  may be formed as one body. Generally, a barrier layer is formed on the bottom of a hole of the anodizing alumina template. The barrier layer is removed by dry etching so that the hollow rods  140   a  of the porous insulating layer  140  may be perforated and the substrate  120  may be exposed to the outside via the hollow rods  140   a.    
     As another example, when the porous insulating layer  140  is formed of a polymer nano template, the porous insulating layer  140  is separately formed from the substrate  120  and is bonded to the substrate  120 . 
     The nano structure  130  may be formed of a conductive material and is rod-shaped in the plurality of hollow rods  140   a  of the porous insulating layer  140 . A material that is grown in a hollow hole of a nanoporous template and has conductivity may be used to form the nano structure  130 . For example, the nano structure  130  may be formed by filling a conductive metal such as copper (Cu), nickel (Ni) or a nickel chrome alloy in the hollow rods  140   a  of the porous insulating layer  140  by using electroplating. The nano structure  130  may be formed by growing a material in the hollow hole of the nanoporous template or by growing carbon nanotubes. 
     As described above, the bottom surface of the hollow rods  140   a  of the porous insulating layer  140  may be removed so that the nano structure  130  directly contacts the substrate  120  via the hollow rods  140   a  of the porous insulating layer  140 . Thus, the nano structure  130  is electrically connected to the substrate  120 . The nano structure  130  may serve as an electrode in which a corona discharge is incurred and may be protected mechanically and electrochemically due to the porous insulating layer  140 . 
     The nano structure  130  may be formed by filling a material in each of the hollow rods  140   a  of the porous insulating layer  140  or may protrude from the hollow rods  140   a , as illustrated in  FIG. 7 . As occasion demands, a nano structure  130 ′ may be formed by filling a material only in portions of each of the hollow rods  140   a  of the porous insulating layer  140 , as illustrated in  FIG. 8 . The tip portion  150  is in the shape of a continuous strip. However, the present invention is not limited thereto, and the tip portion  150  may have various shapes. 
     Referring to  FIG. 7 , the porous insulating layer  140  may have a height h 1 , and the nano structure  130  may have a height h 2 . The height h 1  and the height h 2  may be same. It is possible that the height h 1  and the height h 2  are different. The holes of the porous insulating layer  140  may be different shape from the hollow rods  140   a  or the nano structure  130 . And the tip portion  150  may protrude from a surface of the porous insulating layer  140  in the third direction z. When a material is filled only in portions of each of the hollow rods  140   a  of the porous insulating layer  140  to form the nano structure  130 , the nano structure  130 ′ may be formed by a first portion made of the material with a height h 2  and a second portion made of another material with a height of hd with respect to a height h 1  of the hole (or the hollow rod  140   a ). The another material may be an empty space which is not filled with the material. 
       FIGS. 9A and 9B  illustrate tip portions  151  and  152  of the charging electrode  110  according to an embodiment of the present general inventive concept. Referring to  FIG. 9A , the tip portion  151  is formed in a dotted line in which a group of nano structures constitutes islands  151   a , and the islands  151   a  may be non-continuous islands and may be arranged in a row at equal intervals. Each of the islands  151   a  includes the porous insulating layer  140  and the nano structure  130  disposed in the plurality of hollow rods  140   a  of the porous insulating layer  140 , as illustrated in  FIG. 6 . The hollow rods  140   a  may be represented by dots of the respective islands  151   a  and may be disposed on corresponding dotted lines of the respective islands  151   a . In this case, the islands  151   a  may be spaced-apart from each other by a predetermined distance Sh. The size of each of the islands  151   a  or the predetermined distance Sh between the islands  151   a  may vary according to a design of the charging device  100  of  FIG. 1 . When the nano structure  130  has the arrangement of the non-continuous islands  151   a , due to an effect that occurs at a boundary surface between the islands  151   a , the strength of an electric field near each of the islands  151   a  is relatively increased as compared to the same charging voltage. Since a charging operation of the charging device  100  can be performed with a charging start voltage, and the strength of an electric field near each of the islands  151   a  is relatively increased, the charging operation can be performed with a low charging start voltage. As such, the charging start voltage may be reduced. Furthermore, an inclination of the graph representing the strength of the electric field is increased so that an area in which a discharge product such as ozone or a nitrogen oxide is generated may be reduced. On the other hand, in the current embodiment, although the tip portion  151  is not formed continuously, charging may be uniform by using the grid electrode ( 190  of  FIG. 1 ). In this case, the grid electrode  190  may be connected to the nano structure  130  while passing an area in which the dotted line shape of the tip portion  151  is projected, or while passing an area other than the area in which the dotted line shape of the tip portion  151  is projected. For example, a surface on which the grid electrode  190  is disposed in a plane formed in the first and second directions x and y is installed in the charging electrode  110  to maintain a predetermined distance in the third direction from the islands  151   a  or to be spaced apart from a surface of the islands  151   a  by the predetermined distance. In this case, the grid holder  180  may be disposed to hold or support grids of the grid electrode  190  to have a gap between the grids and a top surface of the islands  151   a  which protrude from the substrate  120  in the third direction z. 
     Referring to  FIG. 9B , the tip portion  152  may include islands  152   a  and  152   b  which include a group of nano structures to be arranged in two rows. The two-row islands  152   a  and  152   b  are alternately arranged while being separated from each other by a predetermined distance Sv in the second direction y. In each of the two-row islands  152   a  and  152   b , islands  152  or  15   b  can be spaced-apart from each other by a distance, fore example, distance Sh in the first direction x. In the current embodiment, as the two-row islands  152   a  and  152   b  are alternately arranged, the non-uniformity of charging caused by non-continuous arrangement of the one-row islands  152   a  may be reduced or prevented by the other-row islands  152   b  so that uniformity of charging may be obtained. Furthermore, the charging device  100  of  FIG. 1  may further include the grid electrode  190  so that uniformity of charging may be improved. 
     Next, the grid electrode  190  and a structure to support the grid electrode  190  will be described with reference to  FIGS. 10 through 13 .  FIG. 10  is an enlarged view of a region C of  FIG. 2 , and  FIG. 11  is a partial top view of the region C of  FIG. 2 .  FIG. 12  is an enlarged view of a region D of  FIG. 2 , and  FIG. 13  is a side view of the region D of  FIG. 2 . 
     Referring to  FIGS. 2 through 13 , the grid electrode  190  may be formed in such a way that the area in which the tip portion  150  is projected is opened. However, the present invention is not limited thereto, and the grid structure  190  may be connected to the charging electrode  110  while passing an area in which the tip portion  150  is projected. 
     The grid electrode  190  is separated from the tip portion  150  of the charging electrode  110  by a distance of several tens of μm to several mm. The spacer  170  is disposed at both ends of the substrate  120  and supports the grid electrode  190 . The spacer  170  is formed of material having an insulation property.  FIG. 1  illustrates the case that the spacer  170  is formed on the top surface of the substrate  120 . However, the present invention is not limited thereto. For example, the porous insulating layer  140  may be formed on all portions of the top surface of the substrate  120 , and the spacer  170  may be disposed on the porous insulating layer  140 . 
     In order to separate the grid electrode  190  from the tip portion  150  at equal intervals, the grid holder  180  may be disposed in the middle of the grid electrode  190  to support the grid electrode  190 . An optimum distance between the grid electrode  190  and the tip portion  150  may be determined according to a diameter of the nano structure ( 130  of  FIG. 1 ) or a distance between the nano structures  130 . 
     The charging device  100  of  FIG. 1  may finely adjust the distance between the grid electrode  190  and the tip portion  150  by using the spacer  170  and the grid holder  180 . As such, the charging device  100  may be designed according to a fine charging characteristic of the nano structure  130  of the tip portion  150 . 
     The grid electrode  190  may include a structure having a frame formed with grids  191   a  and  191   b  to define one or more openings  190   a  and also formed with grids  191   c  and  191   d  to define one or more openings  190   b  with the grids  191   a  and  191   b.    
     The grid holder  180  may include a first portion  180   a , a second portion  180   b  extended from the first portion  180   a  to surround the substrate and/or a portion of the nano electrode  130 , and a third portion  180   c  to hold and support the grids. The third portion  180   c  may include a distal end or clip-structure  180   d  to support the grids with respect to the nano electrode  130 . 
     Next, an operation of the charging device  100  will be described with reference to  FIG. 1 . 
     The charging device  100  charges the layer  15  of the object  10  by using a corona discharge. The corona discharge is a phenomenon in which insulation of a dielectric substance is partially destroyed in an area to which a predetermined strength of an electric field is applied, gaseous particles are ionized and a current flows. In the charging device  100 , the nano structure  130  including a plurality of sharp nano rods  130  constitutes a tip portion of the charging electrode  110 . Thus, the strength of an electric field is particularly increased near ends of the nano rods  130   a . Thus, the corona discharge occurs near the ends of the nano rods  130   a . The ionized gaseous particles are moved to the object  10  due to an electric field applied between the charging electrode  110  and the object  10  and collide with the surface of the object  10 , i.e., the layer  15 , so that the layer  15  may be charged. On the other hand, the ionized gaseous particles that pass through the grid electrode  190  may be controlled according to the adjustment voltage Vg applied to the grid electrode  190 , and a path along which the ionized gaseous particles are moved may be adjusted so that uniformity of charging may be improved. To this end, the charging voltage Ve is applied to the charging electrode  110 , and the adjustment voltage Vg that is lower than the charging voltage Ve is applied to the grid electrode  190 . On the other hand, a lower voltage than the charging voltage Ve and the adjustment voltage Vg is applied to the support  11  of the object  10 . For example, a charging voltage of −5000 V is applied to the charging voltage  110 , a voltage of −600 V is applied to the grid electrode  190 , and a voltage of −500 V is applied to the support  11  of the object  10 . 
     The layer  15  of the object  10  is moved in the second direction y that is a widthwise direction of the charging electrode  10 . For example, when the object  10  has a cylindrical shape such as a photosensitive drum, as the object  10  is rotated, the layer to be charged  15  may be moved in a circumference direction. As described above, due to the electric field applied between the charging electrode  110  and the object  10 , the ionized gaseous particles collide with the layer  15  to charge the layer  15 , and as the layer  15  is moved, all portions of the layer  15  may be charged. 
     In the charging device  100  of  FIG. 1 , the tip portion of the charging electrode  110  includes the plurality of sharp nano rods  130   a . Thus, a discharge characteristic may be improved so that a corona discharge may easily occur even at a low charging voltage. Also, although the amount of charging of each of the nano rods  130   a  is small, the nano structure  130  is formed of the plurality of nano rods  130   a  so that a sufficient amount of charging may be performed. 
       FIGS. 14 through 20  illustrate a charging device according to another embodiment of the present invention. 
       FIG. 14  is a perspective view of a charging electrode disposed in a charging device according to another embodiment of the present general inventive concept, and  FIG. 15  is a partial enlarged view of a region E of  FIG. 14 .  FIG. 16  is a partial enlarged view of a region F of  FIG. 15 , and  FIG. 17  illustrates a protrusion according to an embodiment of the present general inventive concept. Referring to  FIGS. 14 through 17 , the charging device according to the present embodiment includes a substrate  220  including a plurality of protrusions  221 , and a charging electrode  210  including a tip portion  250  disposed on each of the plurality of protrusions  221 . 
     The substrate  220  may be formed of a conductive material such as a metal, for example, aluminum (Al). The plurality of protrusions  221  are formed on a top surface  220   a  of the substrate  220  in two rows and extend in the first direction x. 
     The plurality of protrusions  221  are formed of a conductive material. The plurality of protrusions  221  extend from the top surface  220   a  of the substrate  220  and may be formed with the substrate  220  as a one body. As occasion demands, the plurality of protrusions  221  may be separately formed from the substrate  220  and may be bonded to the top surface  220   a  of the substrate  220 . An area of an upper portion  221 - 2  ( 221 ′- 2 ) of each of the plurality of protrusions  221  is the same as or smaller than an area of a lower portion  221 - 1  ( 221 ′- 1 ) of each of the plurality of protrusions  221 . For example, the plurality of protrusions  221  may have a truncated conical shape, as illustrated in  FIG. 10 . As illustrated in  FIG. 17 , a protrusion  221 ′ may have a truncated reverse pyramidal shape. In addition, the protrusion  221 ′ may have a cylindrical shape, a square pillar shape or other various shapes. 
     A tip portion  250 ,  250 ′ is disposed at a top surface of each of the plurality of protrusions  221 . The structure of the tip portion  250  may be substantially the same as the structure of the tip portion  150  described with reference to  FIGS. 6 through 8 . In other words, the tip portion  250  has a structure in which a nano structure is formed in hollow rods of a porous insulating layer and is supported by the porous insulating layer. Also, the hollow rods of the porous insulating layer are perforated so that the nano structures formed in the hollow rods of the porous insulating layer may be electrically connected to the protrusions  221 . The plurality of protrusions  221  and the tip portion  250  may be formed by using an anodizing alumina template, as described above. For example, the top surface  220   a  of the substrate  220  is first oxidized to form the anodizing alumina template and then, the nano structure is formed in hollow holes of the anodizing alumina template. Next, the area of the tip portion  250  is patterned in the anodizing alumina template in which the nano structure is filled, and then, the top surface  220   a  of the substrate  220  in portions other than the tip portion  250  is etched to form the protrusions  221  and the tip portion  250  disposed on the protrusions  221  may be formed at the top surface  220   a  of the substrate  220 . The plurality of protrusions  221  may be formed at the top surface  220   a  of the substrate  220  by forging or die casting as well as photolithography, and ends of the protrusions  221  may be planarized, and then, the tip portion  250  that is separately manufactured may be attached to the protrusions  221 . 
       FIGS. 18 and 19  are top and side views of the region E of  FIG. 14 , respectively, which illustrate the arrangement of the plurality of protrusions  221 . Referring to  FIGS. 18 and 19 , the plurality of protrusions  221  includes first row protrusions  221   a  and second row protrusions  221   b . The tip portions or center portions of the first row protrusions  221   a  are separated from one another at a pitch interval P, and the tip portions or center portions of the second row protrusions  221   b  are separated from one another at the pitch interval P. Also, the first row protrusions  221   a  and the second row protrusions  221   b  are alternately arranged, as illustrated in  FIG. 13 . As the protrusions  221  are alternately arranged in two rows in this manner, the tip portions  250  that are located at ends of each of the plurality of protrusions  221  are compactly arranged in a lengthwise direction (direction x of  FIG. 14 ). Thus, charging may be uniformly performed by using the tip portions  250  in the lengthwise direction (direction x of  FIG. 14 ). Furthermore, the charging device according to the present embodiment may further include the grid electrode  190  described with reference to  FIGS. 1 and 2  so that uniformity of charging may be further improved. 
     It will be understood by those of ordinary skill in the art that the greater a stepped portion on the surface of an electrode to which a voltage is applied, the greater an electric field near the electrode in which the stepped portion is formed. In the charging electrode  210  according to the present embodiment, as the tip portion  250  includes the above-described nano structure and has a protrusion structure in which the tip portion  250  is formed on the protrusions  221  of the substrate  220 , an electric field that is generated due to the applied charging voltage is increased near the tip portion  250 . Thus, a charging distance between the tip portion  250  and the object to be charged  10  may be sufficiently increased, and for example, a charging distance of 7 mm may be obtained. 
     Furthermore, as the charging electrode  210  has a protrusion structure, the distribution of electric fields between the tip portion  250  and the object  10  may be very steep or variable so that the amount of discharge products may be reduced. In other words, when the ratio of a maximum electric field strength Ea at an end of the nano structure in the tip portion  250  to a minimum electric field strength Eb in the object  10  is an electric field concentration factor α, due to the protrusion structure of the charging electrode  210 , the value of the electric field concentration factor α is increased. 
       FIG. 20  illustrates the distribution of electric fields between the charging electrode  210  and the object to be charged  10 . A position having a distance value of 0 denotes a tip portion ( 250  of  FIG. 15 ), and a distance R 3  denotes the surface of an object such as a photoreceptor ( 10  of  FIG. 1 ). Referring to  FIG. 20 , the maximum electric field strength E a  occurs at the position  0  that is the closest to the tip portion  250 , i.e., the end of the nano structure. An electric field strength is rapidly decreased according to a distance from the tip portion  250 , and a nearly uniform minimum electric field strength E b  occurs near the object to be charged  10 . The maximum electric field strength E a  has at least a value of 3 MV/m that is an ionization electric field strength E 1  of air, and the minimum electric field strength E b  has a smaller value than 1.25 MV/m that is a dissociation electric field strength E 2  of air. In ionization regions  0  to R 1  having an electric field strength between the maximum electric field strength E a  and the ionization electric field strength E 1 , gaseous molecules of air are ionized. The ionized gaseous molecules are moved to the object  10  due to an electric field applied between the charging electrode  210  and the object  10 . On the other hand, in dissociation regions R 1  to R 2  having an electric field strength between the ionization electric field strength E 1  and the dissociation electric field E 2 , portions of the ionized gaseous molecules are combined to generate a discharge product such as ozone or a nitrogen oxide. Thus, the volume of the dissociation regions R 1  to R 2  may be reduced so that the amount of the discharge product may be reduced. In the current embodiment, as the charging electrode  210  has the protrusion structure, an inclination of the graph representing an electric field strength near the tip portion  250  becomes steep so that the volume of the dissociation regions R 1  to R 2  may be reduced. In particular, since a border region of the tip portion  250  disposed at the end of each protrusion  221  is stepped or has a step-like structure, an inclination of the graph representing an electric field strength is more steep than in a central region of the tip portion  250 . The inclination of the graph representing an electric field strength may be indicated by the value of the electric field concentration factor α. The electric field concentration factor α may be varied according to the shape of the end of the protrusion  221  or an interval of the arrangement of the protrusions  221 . In the current embodiment, the charging electrode  210  has the protrusion structure so that the value of the electric field concentration factor α near the central region of the tip portion  250  is equal to or greater than 3. Thus, generation of the discharge product may be prevented. 
       FIGS. 21 through 26  illustrate a charging device according to another embodiment of the present inventive concept. 
       FIG. 21  is a perspective view of a charging electrode  310  disposed in a charging device according to another embodiment of the present general inventive concept, and  FIG. 22  is a partial enlarged view of a region G of  FIG. 21 .  FIG. 23  is a partial enlarged view of a region H of  FIG. 22 , and  FIG. 24  illustrates a protrusion according to an embodiment of the present general inventive concept. The charging device according to the present embodiment is substantially the same as the charging device described with reference to  FIG. 14  except for the structure of protrusions, and thus, only the structure of the protrusions will be described below. 
     Referring to  FIGS. 21 through 23 , a substrate  320  is formed of a conductive material, and the charging device has a two-dimensional arrangement structure in which a plurality of protrusions  321  are arranged in eleven (11) rows and extend from a top surface  320   a  of the substrate  320  in a first direction x. 
     The plurality of protrusions  321  are formed of a conductive material and may extend from the top surface  320   a  of the substrate  320  and may be formed with the substrate  320  as one body. As occasion demands, the plurality of protrusions  321  may be separately formed from the substrate  320  and may also be bonded to the top surface  320   a  of the substrate  320 . An area of an upper portion  321 - 2  of each of the plurality of protrusions  321  is the same as or smaller than an area of a lower portion  321 - 1  of each of the plurality of protrusions  321 . For example, the plurality of protrusions  321  may have a truncated conical shape, as illustrated in  FIG. 23 . In addition, the protrusions  321  may have a truncated reverse pyramidal shape, a cylindrical shape or a square pillar shape. 
     A tip portion  350  is disposed at a top surface of each of the plurality of protrusions  321 . The structure of the tip portion  350  may be substantially the same as the structure of the tip portion  150  described with reference to  FIGS. 6 through 8 . In other words, the tip portion  350  has a structure in which nano structures are formed in hollow rods of a porous insulating layer and the nano structures are supported by the porous insulating layer. Also, the hollow rods of the porous insulating layer are perforated so that the nano structures formed in the hollow rods of the porous insulating layer may be electrically connected to the protrusions  321 . The plurality of protrusions  321  and the tip portion  350  may be formed by using an anodizing alumina template, as described above. The structure of the protrusions  321  and the tip portion  350  may be understood as a reduction shape of the protrusions  221  and the tip portion  250  described with reference to  FIGS. 16 and 17 . 
     Furthermore, the protrusion structure of a charging electrode  310  is not limited to the shape described above, and there may not be an additional nano structure.  FIG. 24  illustrates a protrusion  321 ′ according to an embodiment of the present general inventive concept. Referring to  FIG. 24 , the protrusion  321 ′ extends from the substrate  320  and constitutes a tip portion of the charging electrode  310 . As described above, the protrusion structure of the charging electrode  310  according to the present embodiment is an array structure in which protrusions  321 ′ are two-dimensionally arranged in the top surface  320   a  of the substrate  320 . Thus, the protrusions  321 ′ may serve as a tip that is substantially small. For example, when a width of the top surface  320   a  of the substrate  320  in a second direction (y of  FIG. 21 ) is about 1 mm, if the protrusions  321 ′ are arranged in 10 or more rows in the second direction y, each of the protrusions  321 ′ is less than 100 μm, and an end of each of the protrusions  321 ′ becomes smaller so that an electric field strength may be increased at the end of each of the protrusions  321 ′ and the value of an electric field concentration factor α may be set to 3 or more. 
       FIGS. 25 and 26A  are top and side views of the region G of  FIG. 21 , respectively, which illustrate the structure of arrangement of the plurality of protrusions  321 . Referring to  FIGS. 25 and 26 , the plurality of protrusions  321  include first through eleventh row protrusions  321   a ,  321   b ,  321   c , . . . , and  321   k . A direction L 1  in each of the rows of the first through eleventh row protrusions  321   a ,  321   b ,  321   c , . . . , and  321   k  is parallel to the first direction x. The tip portions of the first through eleventh row protrusions  321   a ,  321   b ,  321   c , . . . , and  321   k  are separated from one another in the first direction x at a pitch interval P. On the other hand, each of the rows of the first through eleventh protrusions  321   a ,  321   b ,  321   c , . . . , and  321   k  is arranged to have a difference Ps of the arrangement of neighboring rows with respect to the first direction x. The difference Ps may be obtained by using Equation 1: 
       Ps=P/N  (1),
 
     , where N is the number of rows in which the protrusions  321  are arranged. In the current embodiment, N is 11. As a result, a segment L 2  that connects the adjacent protrusions  321  in the second direction y may not be exactly consistent with the second direction y and may be slightly inclined or having an angle with the second direction y. Also, the first through eleventh row protrusions  321   a ,  321   b ,  321   c , . . . , and  321   k  are arranged in the first direction x at one pitch interval P, as illustrated in  FIG. 26 . In other words, the protrusions  321  are arranged in the first direction x at one pitch interval P. In this way, the protrusion  321  has one pitch interval P with the adjacent protrusion  321 . However, when viewed from an object to be charged such as a photoreceptor ( 10  of  FIG. 1 ), the protrusions  321  are arranged in the first direction x at a density that is eleven times greater than a density at which the first row protrusions  321  are arranged. Thus, the tip portions  350  are more compactly arranged in the first direction x so that the object to be charged  10  may be charged uniformly. In the charging device according to the present embodiment, a very high electric field strength near the tip portion  350  of the charging electrode  310  may be obtained, and uniformity of compact charging may be obtained due to the two-dimensional arrangement structure. As such, a charging distance between the tip portion  350  of the charging electrode  310  and the object to be charged  10  may be set to be less than 1 to 4 mm in consideration of a design tolerance. In addition, a charging start voltage may be reduced, and the amount of ozone generated may be greatly reduced. 
     Referring to  FIG. 26B , a plurality of first groups of tip portions are disposed on corresponding lines  321   a , . . .  321   k . For example, adjacent ones of the first group tip portions  350 - a   1  and  350 - a   2  are disposed on the line  321   a  in a direction, and another adjacent ones of the first group tip portions  350 - k   1  and  350 - k   2  are disposed on the line  321   k  in the direction. The tip portions (or center portion of the tip portions) of the first group are spaced apart from each other by an interval P. The second group tip portions  350 - a   1 ,  350 - b   1 ,  350 - k   1  are disposed in another direction within the interval P. The second group tip portions are spaced apart from each other by another interval. The another direction and a line connecting the tip portions  350 - a   2  and  350 - k   2  are disposed to have an angle. Accordingly, the tip portion  350 - b   1  is disposed to have a distance Pb=P−P/N with the line. Here, P is an interval, and N is the number of the second group tip portions disposed in the interval P. One of the tip portions has a distance Pk- 1 =P−(N−1)P/N, for example. The tip portions  250 - a   1 ,  350 - b   1 , . . .  350 - k   1  may have a distance with the line by the above describe distance according to a distance from the first group tip portion  350 - a   1  or  350 - a   2 . 
     In addition, in the charging device according to the present embodiment a sufficient uniformity of charging may be obtained without including a grid electrode due to the compact two-dimensional arrangement structure of the plurality of protrusions  321 , and a grid electrode ( 190  of  FIG. 2 ) may be additionally included in the charging device so as to more easily control a charging voltage. 
     In the present embodiment, the protrusions  321  have the two-dimensional arrangement structure in which the protrusions  321  are arranged in 11 rows. However, the present invention is not limited thereto. For example, the protrusions  321  may have a two-dimensional arrangement structure in which the protrusions  321  are arranged in the number of rows that is smaller or greater than 11 and may also have an irregular arrangement structure. 
       FIG. 27  is a schematic view of an electrophotographic image forming apparatus including a charging device, according to an embodiment of the present general inventive concept. Referring to  FIG. 27 , the electrophotographic image forming apparatus includes an image developing unit  501  which includes a photosensitive drum  510 , a charging device  520 , a light scanning unit  530 , a developing unit  540 , a cleaning unit  560 , and an electrostatic charge-removing unit  570 , and an image transfer unit  502  which includes an intermediate transfer belt  550 , first and second transfer rollers  551  and  580 , and a fusing unit  590 . 
     In order to print a color image, the photosensitive drum  510 , the charging device  520 , the light scanning unit  530 , the developing unit  540 , the cleaning unit  560 , and the electrostatic charge-removing unit  570  may be provided according to color. The photosensitive drum  510  disposed for color such as black (K), magenta (M), yellow (Y) or cyan (C) is an example of a photoreceptor and is a photosensitive layer formed at an outer circumferential surface of a cylindrical metal pipe to a predetermined thickness. A photosensitive belt may be employed as the photoreceptor. The outer circumferential surface of the photosensitive drum  510  is a surface to be scanned. The charging device  520  charges the surface of the photosensitive drum  510  at a uniform electric potential, and a charging device according to the afore-mentioned embodiments may be used as the charging device  520 . A charging voltage is applied to the charging device  520 . The light scanning unit  530  scans a light beam that is modulated according to image information on the photosensitive drum  510  in a main scanning direction. As the light beam is scanned on the surface to be scanned of the photosensitive drum  510  of which surface is charged by the charging device  520  at a uniform electric potential, an electrostatic latent image is formed. In this case, the surface to be scanned is moved in an auxiliary scanning direction as the photosensitive drum  510  is rotated, and the light scanning unit  530  scans the light beam on the surface to be scanned of the photosensitive drum  510  in the main scanning direction while being synchronized with a horizontal synchronous signal so that a two-dimensional electrostatic latent image may be formed on the surface to be scanned of the photosensitive drum  510 . 
     An electrostatic latent image that corresponds to image information about black (K), magenta (M), yellow (Y), and cyan (C), respectively, is formed on each of four photosensitive drums  510 . Each of the four developing units  540  supplies a toner of color such as black (K), magenta (M), yellow (Y), and cyan (C) to the photosensitive drum  510  to develop a toner image of color such as black (K), magenta (M), yellow (Y) or cyan (C). The intermediate transfer belt  550  travels in contact with the four photosensitive drums  510 . The toner images of color such as black (K), magenta (M), yellow (Y), and cyan (C) formed on each of the photosensitive drums  510  are transferred onto the intermediate transfer belt  550  due to a first transfer bias voltage applied to the first transfer roller  551  and are overlapped on the intermediate transfer belt  550 . A drum type intermediate transfer belt, instead of the intermediate transfer belt  550  may be employed. The remaining toner images after a transfer operation are removed by the cleaning unit  570 . Also, an electrostatic-removing operation is performed on the surface of the photosensitive drum  510  in which the transfer operation is completed, so that a developing operation of one cycle is performed. The toner image transferred onto the intermediate transfer belt  550  is transferred onto a recording medium  600  on the intermediate transfer belt  550  due to a second transfer bias voltage applied to the second transfer roller  580 . The toner image transferred onto the recording medium  600  is fused on the recording medium  600  by the fusing unit  590  due to heat and pressure so that a printing operation may be completed. 
     In the above-described embodiments, the photosensitive drum  10  is used as an object to be charged. However, the present invention is not limited thereto, and the photosensitive drum  10  may be used in a transfer operation. 
     In the electrophotographic image forming apparatus including the charging device according to the present invention, the above-mentioned fusing unit is used so that a charging start voltage may be reduced and the amount of total current required in discharging may be reduced. Also, the amount of a discharge product such as ozone or a nitrogen oxide may be reduced, and uniformity of charging may be improved. 
     Although a few embodiments of the present invention have been shown and described, it would be appreciated by those skilled in the art that changes may be made in these embodiments without departing from the principles and spirit of the general inventive concept, the scope of which is defined in the claims and their equivalents.