Patent Publication Number: US-7715760-B2

Title: Charging device, and process cartridge and image forming apparatus using the same

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a charging device, a process cartridge and an image forming apparatus, and more particularly to a charging device which charges an image bearing member by electron discharging, a corresponding process cartridge and a corresponding image forming apparatus. 
   2. Discussion of the Background 
   An image forming apparatus based on electrophotographic process is known to function as a printer, a facsimile machine, a photocopier, a plotter, and a multi-functional machine having functions of a printer, a facsimile machine and a photocopier. In this electrophotographic process, corona charging is widely adopted to uniformly charge an image bearing member to form a latent electrostatic image thereon. 
   This corona charging system is as follows: an electroconductive case electrode is provided around a wire electrode formed of platinum or tungsten having a diameter of from about 50 to about 200 μm or an electrode having a needle form made of a stainless material; DC or AC high voltage is applied between the electrode and the case to ionize air molecules around the electrode; and the image bearing member is charged by the ionized molecules. In this system, uniform charging can be performed from a distance. 
   However, in the corona charging system, air is ionized so that corona products such as ozone and nitrogen oxides are produced. The amount of the products for both of ozone and nitrogen oxides, is known to reach as high as 4 to 10 ppm after 60 minute charging. 
   It is known that when the density of ozone accumulated in an image forming apparatus is high, the surface of an image bearing member is oxidized, resulting in deterioration of the photosensitivity of the image bearing member and the charging power. Therefore, degraded images are obtained (refer to “Development of Corona Charger for the Reduction of the Bad Influence of Ozon on a Photoconductor” (The Imaging Society of Japan, ISJ), authored by Hisashi MYOCHIN, et. al., Vol. 31, 1, published in 1992). In addition, deterioration of devices other than an image bearing member is accelerated, resulting in problems such as short life of the devices. 
   Further, such nitrogen oxides cause the following problems. That is, the nitrogen oxides produce nitric acid in reaction with moisture in the air and metal nitrate in the reaction with a metal. These products have a high electric resistance in a low moisture environment but a low electric resistance in a high moisture environment when reacting with moisture in the air. Therefore, when a thin layer of nitric acid or metal nitrate is formed on the surface of an image bearing member, abnormal images such as a flowing image are obtained. This is because nitric acid or a salt thereof absorbs moisture so that the resistance of the surface of the image bearing member is low, resulting in destruction of a latent electrostatic image on the surface of the image bearing member. 
   Further, nitrogen oxides still accumulate in the air in the image forming apparatus without dissolving after discharging. The compound produced from the nitrogen oxides attaches to the surface of the image bearing member even when charging is not performed, that is, during idle periods of the process. Furthermore, the compound infiltrates from the surface to the inside of the image bearing member over time. This causes deterioration of the image bearing member. 
   In this case, the attachment on the surface of the image bearing member is removed by slightly scraping the image bearing member at a time during cleaning. However, this method involves new problems such as cost increase and deterioration over time. 
   In the corona charging, the applied voltage is significantly high, that is, from 4 to 10 kV, because the charging is performed from a distance. In addition, the charged voltage depends on the charging time. Therefore, when the linear velocity of an image bearing member is high, it is necessary to broaden the width of a case electrode along the rotation direction of the image bearing member. Therefore, the size reduction of an image forming apparatus having a high printing speed is difficult. 
   As another charging device, a charging system using a roller disposed in the vicinity of an image bearing member is now widely used. In the vicinity type roller charging, an image bearing member is charged such that an AC or DC bias is applied to between an image bearing member and a charging device (charging roller) disposed in the vicinity of the image bearing member to cause discharging in the space therebetween. In this charging system, the charging phenomenon based on Paschen&#39;s law is utilized. That is, a desired voltage is obtained by forming a voltage difference by a discharging starting voltage against the desired voltage. 
   In the AC bias system, the direction of the electric field formed between an image bearing member and a charging device disposed in the vicinity thereof alternates with time. Therefore, discharging and reversed discharging are repeatedly performed. Charging is evened out by discharging and reversed discharging in the AC bias system, resulting in uniform charging. However, there is a disadvantage that hazard to an image bearing member by discharging is extremely large. 
   The electron is provided to an image bearing member by a charging device involving Paschen discharging. As a result, hazard is inevitable. For example, products produced by discharging are attached to the surface of an image bearing member or the surface of an image bearing member is oxidized by an active air produced by discharging. 
   To deal with this drawback, as mentioned above, the surface of an image bearing member is minutely scraped at a time to reduce the deterioration of image quality over time. On the other hand, scraping the surface of an image bearing member is equal to attrition. It is naturally preferred to avoid such scraping from a long-term point of view. However, this scraping has a trade-off relationship with protection against image deterioration caused by the hazard to an image bearing member mentioned above. Therefore, it is difficult to find a fundamental solution to this drawback. 
   Further, there is another charging system, which is a contact type charging device. In this system, a charging member is brought into contact with an image bearing member to charge the image bearing member. For example, a charging member having a roller form charges an image bearing member by being driven by the image bearing member while in contact therewith. When compared with the corona charging system mentioned above, the contact type charging system has advantages such that the amount of ozone produced after 60 minute charging using DC is as small as 0.01 ppm, the applied voltage is low so that the cost of power is small and it is easy to design electric insulation. 
   As the contact type charging system, as described in unexamined published Japanese Patent Applications Nos. (hereinafter referred to as JOP) S57-178257, S56-104351, S58-40566, S58-139156, and S58-150975, there are methods in which discharging according to the interpretation of Paschen&#39;s law is performed at the contact portion or a narrow space formed in the vicinity thereof to charge an image bearing member. It is possible to accelerate uniform charging by applying a DC voltage not less than the charge starting voltage to an electroconductive portion or applying an oscillation voltage in which an AC voltage is overlapped with a DC voltage corresponding to the desired charging voltage, which is detailed in JOP 63-149669. 
   As mentioned above, when an AC voltage is applied, there is an advantage that, since the direction of electric field formed between an image bearing member and a charging device disposed in the vicinity thereof alternates with time, discharging and reversed discharging are repeatedly performed so that charging is evened out by discharging and reversed discharging. However, the amount of electric current increases. Therefore, the amount of ozone and nitrogen oxide produced increases as the electric current increases. Depending on the condition of AC application, the amount of ozone produced reaches almost 3 ppm after 60 minute charging, which is close to that in the corona charging system. 
   On the other hand, as described in JOP H08-106200, there is another method in which the electroconductive member mentioned above to which a voltage is applied is brought into contact with an image bearing member to infuse electrons on the surface of the image bearing member. As the electroconductive member in this method, an electroconductive member having a roller form (charging roller) is typically used in terms of easiness of controlling attachment/detachment and making a form. 
   However, since the charging member is formed of rubber, when, for example, a photocopier is idle for a long time, the roller, which is in contact with an image bearing member, may be transformed. In addition, since rubber is a material which easily absorbs moisture, the electric resistance thereof significantly varies according to changes in environment. Further, rubber requires several kinds of plasticizers and active agents to become elastic and prevent deterioration. Further, a dispersion helping agent is used to disperse electroconductive pigments in a considerable number of cases. That is, since the surface of an image bearing member is an amorphous resin such as polycarbonate and acryl resin, such an image bearing member is extremely weak to the plasticizers, active agents and dispersion helping agents mentioned above. 
   In addition, there are other problems as follows involving the contact-type charging system: when a foreign substance is nipped between a charging member and an image bearing member, the charging member is contaminated, which leads to occurrence of poor charging; and since the roller is in direct contact with an image bearing member, the image bearing member is contaminated overtime, which causes image degradation such as streak in the horizontal direction. 
   As a result, as a superseding technology, the system using an electron discharging material is getting attention. For example, JOP 2003-145826 describes a technology in which the electron discharging element of MIS type and MIM type is used. The electron discharging element has a structure in which an electron discharging layer formed of an insulation layer and a semiconductor material layer or metal material layer is sandwiched between a substrate electrode and a thin layer electrode. 
   JOP 2001-250467 describes a technology using an electron discharging element having a carbon nanotube. The peripheral part of the carbon nanotube is coated with metal or alloyed metal, or at least one compound selected from a nitride, a carbide, a silicide or a boride containing a metal. 
   JOP 2002-279885 describes an electron emission apparatus, a charging device and an image forming apparatus regarding an electron emission apparatus using carbon nanotube, which are capable of stable emission of electron in the atmospheric pressure and with low voltage. 
   JOP 2003-140444 describes a technology using an electron discharging element in which a semiconductor layer is formed between the top electrode and the bottom electrode. An organic compound absorption layer is formed so that an organic compound is absorbed in the semiconductor surface of the semiconductor layer. 
   Further, JOPs 2002-311684 and 2004-327084 describe technologies using an electron discharging element. 
   Among the electron discharging elements mentioned above, the carbon nano materials have been intensively studied. Among these, carbon nanotube has widely studied and it is suggested that carbon nanotube has a high electron discharging power. For example, JOP 2001-250467 describes that durability of carbon nanotubes can be improved by regulating the component for use in the peripheral part thereof and carbon nanotubes can be used as a contact type or non-contact type charging device. 
   However, there is a problem with carbon nano materials stemming from the fact that the carbon nano materials are organic compounds. That is, in the electrophotography system, electrons are discharged into the air. Therefore, carbon nano materials are oxidized by oxygen atoms excited by the discharged electron and dissolved by combustion. Carbon nanotube materials have a short life because of this structural weakness. 
   In addition, electron discharging elements having MIS structure and MIM structure described in JOPs 2003-145826 and 2003-140444 have a problem because the electron discharging property is not sufficient. 
   Because of these reasons, the present inventors recognize that a need exists for a charging device which can perform electron discharging such that corona products are not produced to prevent hazard to an image bearing member and deterioration of material discharging electrons, and a process cartridge and an image forming apparatus using the charging device. 
   SUMMARY OF THE INVENTION 
   Accordingly, an object of the present invention is to provide a charging device which can perform electron discharging such that corona products are not produced to prevent hazard to an image bearing member and deterioration of material discharging electron, and a process cartridge and an image forming apparatus using the charging device. 
   Briefly this object and other objects of the present invention as hereinafter described will become more readily apparent and can be attained, either individually or in combination thereof, by a charging device including an electron discharging device to form a latent electrostatic image on an image bearing member and an electroconductive portion. The electron discharging device contains sp 3  bonding material. 
   It is preferred that, in the charging device mentioned above, the sp 3  bonding material is sp 3  bonding boron nitride. 
   It is still further preferred that, in the charging device mentioned above, the sp 3  bonding boron nitride is selected from the group consisting of sp 3  bonding 5H-BN materials and sp 3  bonding 6H-BN materials. Mixtures of these materials may be used. 
   It is still further preferred that, in the charging device mentioned above, a voltage is applied between the image bearing member and the electron discharging device containing the sp 3  bonding material to discharge electrons from the surface of the sp 3  bonding material and the image bearing member is charged by the electrons or ions generated when the electrons are attached to air molecules. 
   It is still further preferred that, in the charging device mentioned above, the voltage applied between the electron discharging device containing the sp 3  bonding material and the image bearing member is not greater than a discharging limit determined by Paschen&#39;s law. 
   It is still further preferred that, in the charging device mentioned above, the electron discharging device containing the sp 3  bonding material is disposed opposing the surface of the image bearing member with a gap not less than 20 μm. 
   It is still further preferred that, in the charging device mentioned above, the electron discharging device containing the sp 3  bonding material is disposed opposing the surface of the image bearing member with a gap not less than 50 μm. 
   It is still further preferred that, in the charging device mentioned above, the electron discharging device containing the sp 3  bonding material further includes an electroconductive member which is configured to control the voltage of the surface of the image bearing member and disposed between the electron discharging device containing the sp 3  bonding material and the image bearing member. 
   It is still further preferred that, in the charging device mentioned above, the electron discharging device containing the sp 3  bonding material is a thin layer having a thickness not greater than 100 μm formed on the electroconductive portion. 
   It is still further preferred that, in the charging device mentioned above, the electron discharging device containing the sp 3  bonding material is formed such that powder of the sp 3  bonding material is fixed on the electroconductive portion by electroconductively contacting the electroconductive portion. 
   As another aspect of the present invention, an image forming apparatus is provided which includes an image bearing member, the charging device mentioned above to charge the image bearing member to form a latent electrostatic image on the image bearing member, a developing device to develop the latent electrostatic image and a transfer device to transfer the developed image to a transfer material. 
   It is preferred that, in the image forming apparatus, the charging device includes a plurality of independent charging elements and a voltage supplying device is provided such that it is possible to independently set and apply a voltage to each of the plural of independent charging units. 
   It is still further preferred that, in the image forming apparatus, the voltage supplying device applies a controlled voltage such that each charging unit can supply the same amount of charge to the image bearing member. 
   As another aspect of the present invention, a process cartridge is provided which includes an image bearing member, the charging device mentioned above and at least one device selected from the group consisting of a developing device, a transfer device and a cleaning device. The process cartridge is detachably attached to the main body of an image forming apparatus. 
   These and other objects, features and advantages of the present invention will become apparent upon consideration of the following description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Various other objects, features and attendant advantages of the present invention will be more fully appreciated as the same becomes better understood from the detailed description when considered in connection with the accompanying drawings in which like reference characters designate like corresponding parts throughout and wherein: 
       FIG. 1  is a diagram illustrating an example of the electron discharging element for use in the charging device of the present invention; 
       FIG. 2  is a diagram illustrating another example of the electron discharging element; 
       FIG. 3  are diagrams illustrating an embodiment of the charging device of the present invention along the axis direction of an image bearing member; 
       FIG. 4  is a diagram illustrating a lateral side of an electron discharging array for describing an embodiment of the charging device of the present invention; 
       FIG. 5  is a diagram illustrating another embodiment of the charging device of the present invention along the axis direction of an image bearing member; 
       FIG. 6  is a diagram illustrating an embodiment of the image forming apparatus of the present invention; 
       FIG. 7  is a diagram illustrating an example of the process cartridge of the present invention; and 
       FIG. 8  is a diagram illustrating an example of the image forming apparatus having the process cartridge of the present invention. 
       FIG. 9  is a schematic view illustrating a cross section of an example of the charging device of the present invention; 
       FIG. 10  is a schematic view illustrating a laser printer, which is an embodiment of the image forming apparatus of the present invention; 
       FIGS. 11-14  are elevation views of electron discharging layers for use in the charging device of the present invention; 
       FIGS. 15-24  are schematic views illustrating other examples of the charging device of the present invention; and 
       FIG. 25  is a schematic view illustrating a tandem-type full color image forming apparatus which is another embodiment of the image forming apparatus of the present invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   The present invention will be described below in detail with reference to several embodiments and accompanying drawings. 
   First, an electron discharging element functioning as electron discharging device forming the charging device of the present invention is described with reference to  FIGS. 1 and 2 .  FIG. 1  is a diagram illustrating an example of the electron discharging element and  FIG. 2  is a diagram illustrating another example of the electron discharging element. 
   Electron discharging element  1  illustrated in  FIG. 1  is formed by fixing a thin layer  102  of sp 3  bonding material on an electrode  101 . Electron discharging element  11  illustrated in  FIG. 2  is formed by dispersing and fixing powder  122  containing sp 3  bonding material on an electrode  121 . 
   These electron discharging elements  1  and  11  discharge electrons when a voltage is applied to the electrodes  101  and  121  by a power supply (not shown). The discharged electrons generate negative ions by attaching to air molecules such as oxygen molecules, carbon dioxide molecules, nitrogen molecules and molecules thereof to which water is attached. The negative ions charge a body to be charged. 
   How the amount of ozone and nitrogen oxides produced during discharging is reduced when the electron discharging elements  1  and  11  are used is described. 
   In general, an extremely large amount of ozone and nitrogen oxides (NOx) is produced when charging is performed by corona charging. This is because the energy of the electron discharged by a corona wire is not less than 30 eV, which is greater than the dissociation energy of nitrogen molecules on electron collision, i.e., 24.3 eV, and the dissociation energy of oxygen molecules on electron collision, i.e., 8 eV. As a result, air molecules are dissociated by this electron collision therebetween. 
   On the other hand, the energy of the electron generated by an electron discharging element using an sp 3  bonding material is about 6 eV, which is not sufficient to dissociate air molecules. Consequently, nitrogen oxides and ozone are not produced. 
   That is, since corona products (ozone and NOx) are not produced, the electron discharging element using an sp 3  bonding material does not cause hazard to an image bearing member such as attachment of corona products to the surface of the image bearing member and oxidation thereof causing deterioration. The electron discharging material used in the electron discharging element is free from deterioration caused by combustion deriving from oxidation. Therefore, the charging can be stably performed for an extended period of time. In addition, it is possible to obtain a charging voltage sufficient for a body to be charged in a short time even with a low voltage operation. Further, since the system of electron discharging element using an sp 3  bonding material is non-contact type charging, the system is free from deterioration caused by attachment of the toner remaining on an image bearing member after transfer. 
   Next, the sp 3  bonding material is described. Specific examples of the sp 3  bonding materials include sp 3  bonding boron nitrides, for example, sp 3  bonding 5H-BN materials or sp 3  bonding 6H-BN materials or mixtures thereof. These are compounds represented by chemical formula of BN and having hexagonal system 5H type or 6H type multi-angular structure. Specifically, the compound described in Japanese Patent No. (hereinafter referred to as JPP) 3598381 is well known. 
   As a result of the intensive study on application of such sp 3  bonding 5H-BN materials or sp 3  bonding 6H-BN materials to a charging device included in an image bearing member in an image forming apparatus, it is found that it is possible to charge an image bearing member by using sp 3  bonding 5H-BN materials or sp 3  bonding 6H-BN materials as electron discharging materials. The present invention was thus made. 
   Especially, sp 3  bonding 5H-BN materials have the same bonding state as that of diamond and is one of the boron nitrides which is hardest next to diamond. For example, boron nitrides are used to manufacture a crucible. Boron nitrides have excellent thermal resistance and chemical resistance. Therefore, it can be said that boron nitrides are an electron discharging material having a non-conventional durability and a good resistance to high load. 
   These sp 3  bonding 5H-BN materials can be manufactured by irradiating a substrate made of silicon, nickel, etc., with a mixed gas plasma (diboran: boron hydride (B 2 H 6 ), hydrogen, ammonium and argon) together with defocused (slightly concentrated) ultraviolet excimer laser (λ: 193 nm, f: 1030 Hz). In this method, a thin layer of sp 3  bonding 5H-BN is formed and the surface thereof has projections having sharp spindle forms having a length of about 10 μm along with the laser irradiation direction, which function as an electron discharging emitter. 
   When such an electron discharging element using such sp 3  bonding material is manufactured, the sp 3  bonding material thin layer as mentioned above is formed as the surface layer on the electroconductive material (the electrode mentioned above). Therefore, it is possible to make the manufacturing time short while keeping the electron discharging property and reduce cost while limiting the material cost when compared with single crystals. It is also possible to simplify the manufacturing process and reduce cost when a powdered sp 3  bonding material fixed by electroconductively contacting with an electrode, i.e., electroconductive portion, is used in comparison with a single crystal sp 3  bonding material. 
   Next, an embodiment of the charging device of the present invention having such an electron charging element is described with reference to  FIG. 3 .  FIGS. 3A and 3B  are diagrams illustrating the embodiment along the axis direction of an image bearing member. 
   This charging device has a structure in which a plurality of the electron discharging elements  1  (i.e., charging element) are arranged on an electrode  2  formed of, for example, aluminum, without a space. The electron discharging elements  1  are fixedly attached to the electrode  2  with an electroconductive adhesive to form an electron discharging array  10 . A DC power supply  3  applies a voltage to the electron discharging array  10 . 
   To be specific, as mentioned above, the electrode (electroconductive material)  101  (refer to  FIG. 1 ) formed of silicone, nickel, etc., having cross section of 10 mm×5 mm with a thickness of 500 μm is irradiated with a mixed gas plasma (diboran: boron hydride (B2H6), hydrogen, ammonium and argon) together with defocused (slightly concentrated) ultraviolet excimer laser (λ: 193 nm, f: 1030 Hz). As a result, the thin layer  102  (refer to  FIG. 1 ) of sp 3  bonding 5H-BN is formed on the electrode  101  as the electron discharging elements  1 . 
   An image bearing member  201  serving as a body to be charged by this charging device includes an electroconductive substrate  202  and a photosensitive layer  203 . 
   First, the following experiment is performed using an aluminum tube as the electroconductive substrate  202  without the photosensitive layer  203 ; a voltage is applied to the electron discharging element array  10  and the current amount is measured. The current amount is converted into the amount of electron to obtain the theoretical value of the surface charging voltage of an image bearing member. 
   A voltage is applied to the electron discharging array  10  under the following condition: A gap G 1  between the electron discharging array  10  and the electroconductive substrate  202  is maintained to be 20 μm; the electroconductive substrate  202  is not in motion; the electron discharging array  10  is fixed and the width thereof is 5 mm. When −20 V is applied from the power supply  3  to the electron discharging array  10  and the current amount flowing in the electroconductive substrate  202  is measured, it is confirmed that the current amount is sufficient to charge the surface of the image bearing member  201  to be −600 V. 
   Consequently, this charging device is confirmed to have a sufficient charging ability with a relatively extremely low voltage (|20| V in this case) in comparison with a typical charging device. 
   In addition, a voltage is applied to the electron discharging array  10  while the gap G 1  between the electron discharging array  10  and electroconductive substrate  202  is maintained to be 50 μm. When −50 V is applied from the power supply  3  to the electron discharging array  10  and the current amount flowing in the electroconductive substrate  202  is measured, it is confirmed that the current amount is sufficient to charge the surface of the image bearing member  201  (i.e., the electroconductive substrate  202 ) to be −600 V. Further, when the gap G 1  is changed to 100 μm and the power supply  3  applies −100 V, it is confirmed that the current amount is sufficient to charge the surface of the image bearing member  201  (the electroconductive substrate  202 ) to be −100 V. 
   To evaluate the ability of charging an image bearing member, anon-contact type charging is performed under the following conditions: a gap G 2  between the electron discharging array  10  and the image bearing member  201  (i.e., the photosensitive layer  203 ) is maintained to be 20 μm; the linear velocity of the image bearing member  201  is 200 mm/sec; and the electron discharging array  10  is fixed and the width thereof is 5 mm. When −300 V is applied from the power supply  3  to the electron discharging array  10  and the current amount flowing in the electroconductive substrate  202  is measured, it is confirmed that the current amount is sufficient to charge the surface of the image bearing member  201  to be −200 V. 
   The reason the voltage applied to the electron discharging array  10  is set to be −300 V is to prevent discharging by setting a voltage sufficiently lower than the discharging limit voltage of −436V obtained based on Paschen&#39;s law when the gap G 2  is 20 μm. 
   Similarly, another non-contact type charging is performed by changing the gap G 2  between the electron discharging array  10  and the image bearing member  201  (i.e., the photosensitive layer  203 ) to 50 μm. When −300 V is applied from the power supply  3  to the electron discharging array  10 , the surface voltage of the image bearing member  201  is confirmed to reach −100 V. Further, another similar non-contact type charging is performed by fixing the gap G 2  to be 100 μm. When −300 V is applied, the surface voltage of the image bearing member  201  is confirmed to reach −50 V. 
   Judging from the results, when the electron discharging array  10  is used as a charging device to charge the image bearing member  201 , it is confirmed that it is possible to charge the image bearing member  201  with an extremely low voltage in comparison with the case of a typical charging device. The reason the surface charging voltage of the image bearing member  201  does not reach the voltage applied to the electron discharging array  10  as calculated based on the theoretical value is inferred to be that since the photosensitive layer  203  of the image bearing member  201  is insulative, the intensity of the electric field required to discharge electrons is hard to obtain. 
   In addition, it is confirmed that, as the gap G 2  increases, it is harder for the discharged electrons from the electron discharging element  1  to reach the image bearing member  201 . Therefore, to obtain a sufficient charging voltage, it is preferred to provide a grid which accelerates the discharged electrons so that the electrons can reach the image bearing member  201  when the gap G is wide. 
   The voltage applied to the electron discharging element  1  is set such that the voltage difference between the sp 3  bonding material and the surface of the image bearing member is the voltage difference not greater than the discharging limit based on Paschen&#39;s law. Thereby, it is possible to prevent production of ozone and NOx by electrolytic dissociation of air molecules caused by discharging. Further, it is possible to control the charging voltage by preventing the occurrence of Paschen discharging and elongate the life of sp 3  bonding material. 
   As mentioned above, the gap between the sp 3  bonding material and the surface of the image bearing member is set to be not less than 20 μm and preferably not less than 50 μm. Thereby, an allowance for wobbling of an image bearing member and the diameter of a carrier for use in a two component developer is secured. As a result, the electron discharging element is free from scars caused by the contact between the electron discharging element  1  and foreign objects or an image bearing member. Therefore, life of the sp 3  bonding material is elongated. 
   Next, another embodiment of the charging device of the present invention is described with reference to  FIGS. 4 and 5 .  FIG. 4  is a diagram illustrating a lateral side of the charging device in the embodiment and  FIG. 5  is a diagram illustrating the embodiment along with the axis direction of the image bearing member. 
   In this embodiment, the electron discharging array  10  in the embodiment mentioned above is placed in a case  4  having insulation property and three sides with an opening mouth  4   a . A grid  7  made of stainless metal is attached to the opening mouth  4   a  of the case  4  and a power supply  8  applies a voltage to the grid  7 . 
   As the grid  7 , a stainless plate having a honeycomb structure typically adopted in scorotron charging system is used. An electroconductive film through which electrons passes and an electroconductive plate material having holes through which electrons pass can be used. 
   A non-contact type charging is performed as follows: the distance between the electron discharging array  10  and the grid  7  is set to be 50 μm, and the gap between the grid  7  and the image bearing member  201  is set to be 1 mm; the power supply  3  applies −300 V to the electron discharging array  10  and the power supply  8  applies −650 V to the grid  7 ; and the opening mouth  4   a  of the case  4  is disposed opposing the image bearing member  201 . 
   When the linear velocity of the image bearing member  201  is 200 mm/sec, the voltage of the surface of the image bearing member  201  is −600 V. When the applied voltage to the grid  7  is changed, the surface voltage of the image bearing member  201  also varies. When −850 V is applied to the grid  7 , the surface voltage of the image bearing member is about −800 V. In addition, it is confirmed that, when the applied voltage to the grid  7  is reduced, the surface voltage of the image bearing member  201  decreases correspondingly. 
   As described above, by providing an electroconductive material (grid) in the gap between the sp 3  bonding material and the surface of the image bearing member to control the surface voltage of an image bearing member, a desired voltage can be obtained even when the gap between the sp 3  bonding material and the surface of the image bearing member is wide. Thereby, it is possible to perform electron discharging from a distance. Therefore, it is possible to prevent the contamination caused by toner, etc., securely apply the charging voltage, prevent scars caused by contact between the charging member (electron discharging elements) and foreign objects or an image bearing member, and elongate life of the sp 3  bonding material. 
   Next, an embodiment of the image forming apparatus having the charging device of the present invention is described with reference to  FIG. 6 .  FIG. 6  is a diagram illustrating the image forming apparatus. 
   This image forming apparatus has an image bearing member  201  and the image bearing member  201  rotates in the direction indicated by the arrow and has a drum form. Around the image bearing member  201 , there are provided the charging device  211  as described in the second mentioned embodiment having the electron discharging array  10  to charge the surface of the image bearing member  201 , a developing device  213  to develop a latent electrostatic image formed on the charged image bearing member  201  according to the document image by irradiation of a laser beam  212 , a transfer device  216  to transfer the toner image on the image bearing member  201  to a transfer material  215 , a cleaning device  317  to remove the toner remaining on the image bearing member  201  after transfer, and a discharging device  218  to remove the residual charge on the image bearing member  201 . In addition, a fixing device  219  to fix the toner image transferred onto the transfer material  215  is also provided. 
   When the distance (gap G) between the electron discharging array  10  of the charging device  211  and the surface of the image bearing member  201  is set to be 50 μm and −560 V is applied to the electron discharging array  10 , the electrons discharged from the electron discharging array  10  are attached to the image bearing member  201  so that the surface thereof is charged. The image bearing member  201  rotates at 200 mm/sec after charging. A writing device (not shown) forms a latent electrostatic image. Thereafter, the developing device visualizes the latent electrostatic image with a developer such as toner. The toner image formed on the image bearing member  201  is transferred to the transfer material  215  by the transfer device  216 . A subtle amount of toner remains on the image bearing member  201  after the toner image is transferred. The cleaning device  317  removes the remaining toner. Next, the image bearing member  201  is discharged by the discharging device  218  to be ready for the next image formation. 
   A structure without a cleaning device in which remaining toner is retrieved by a developing device is also allowed. 
   The charging device of the present invention having the electron discharging array can charge an image bearing member without producing ozone and NOx as mentioned above. In addition, since the applied voltage by this system can be relatively small in comparison with the case of corona charging and roller type charging, an image forming apparatus using this charging device can save energy. Further, since the electron discharging is performed with a low energy, organic materials such as polycarbonate for use in forming an image bearing member are not oxidized and combusted. As a result, it is possible to reduce the amount of scraping of the image bearing member. 
   Next, another embodiment of the image forming apparatus of the present invention for use in directly forming a latent electrostatic image by the electron discharging element is described. 
   The electron discharging elements  1  are arranged over the length of, for example, 300 mm, in the axis direction of an image bearing member with a density of, for example, 600 dpi. Each electron discharging element  1  is structured to be independently charged. 
   When a print signal is provided, a voltage is applied to each electron discharging element  1  according to the writing signal based on the image. Among all of the electron discharging elements  1 , electrons are discharged from only the electron discharging elements  1  to which the voltage has been applied. The area on the image bearing member disposed opposing the electron discharging elements  1  is charged by the discharged electrons. The area corresponding to the electron discharging elements  1  to which the voltage has not been applied is not charged. Therefore, charged portions and non-charge portions are formed on the image bearing member, which form a latent electrostatic image. That is, a charging device having the electron discharging elements  1  also functions as a writing device for writing a latent image on an image bearing member. 
   Thereafter, the latent image is developed with, for example, toner, and a corresponding visualized image is formed. The toner image formed on the image bearing member is transferred to a transfer material such as paper by a transfer device. A subtle amount of toner remains on the image bearing member after the toner image is transferred. The remaining toner is removed in the next process, i.e., the cleaning process. Next, the image bearing member is discharged by the discharging device to be ready for the next image forming. A structure without a cleaning device in which remaining toner is retrieved by a developing device is also allowed. 
   As described above, in the system in which latent images are directly formed, the size and cost of an image forming apparatus can be reduced because typical two processes, i.e., charging and irradiation processes, can be simplified at a time. In this case, the charging device and the irradiating device function as a charge imparting device. 
   That is, when an image forming apparatus has a plurality of independent charging devices (electron discharging elements using sp 3  bonding materials) which can apply a voltage and a charge application device which can independently set and apply a voltage to the plural of charging devices, it is possible to charge or non-charge a dot on an image bearing member and directly form a desired latent image on the image bearing member during charging. Thereby, an irradiating device for forming a latent image on an image bearing member having photoconductivity is made redundant, which leads to cost reduction of an image forming apparatus. 
   It is preferred to provide a voltage application device which can apply a controlled voltage to the plural of charging elements in a charging device so that each of the plural of charging devices can supply the same amount of charge to an image bearing member. When a latent image is directly formed by multiple minute charging devices (elements) during charging, quality images are not obtained without the uniform voltage. Therefore, to obtain quality images, it is desired to control the amount of charge supplied from each charging device (element) which charges an area corresponding to one pigment to be the same. 
   Next, the process cartridge of the present invention having the charging device of the present invention is described with reference to  FIG. 7 .  FIG. 7  is a diagram illustrating the process cartridge. 
   A process cartridge  300  has a structure in which an image bearing member  301 , a charging device  311  serving as the charging device of the present invention, a developing device  313 , and a cleaning device  217  are integrally combined as a process cartridge. This process cartridge is detachably attached to the main body of an image forming apparatus such as a photocopier and a printer. The process cartridge of the present invention is not limited to this structure but, for example, can have a structure having the charging device, the image bearing member  301 , and at least one of the developing device  313  and the cleaning device  217 . 
   By providing the charging device  311  to the process cartridge detachably attachable to the main body of an image forming apparatus, maintenance property is improved and the process cartridge can be easily replaced. 
   Next, a color image forming apparatus using the process cartridge of the present invention is described with reference to  FIG. 8 .  FIG. 8  is a diagram illustrating the image forming apparatus. 
   This image forming apparatus is a color image forming apparatus having a transfer belt  321  extending in the horizontal direction along which the above-mentioned process cartridges  300 Y,  300 M,  300 C and  300 K are arranged side by side to form each color image of yellow, magenta, cyan and black. The toner images developed on each image bearing member  301  in each process cartridge  300 Y,  300 M,  300 C,  300 K are transferred accordingly to the transfer belt  321  extending in the horizontal direction to which a transfer voltage is applied. 
   Images of yellow, magenta, cyan and black are formed, overlapped on the transfer belt  321  and transferred to a transfer material  323  by a transfer device  322  at one time. The overlapped toner image on the transfer material  323  is fixed by a fixing device (not shown). The process cartridge  300  is described in the order of yellow, magenta, cyan and black in  FIG. 8  but the order thereof is not limited thereto. 
   Typically, a color image forming apparatus has a large size because the color image forming apparatus has multiple image formation portions. In addition, when each unit such as a cleaning device and a charging device is separately broken down and ends its life, the unit is replaced. However, the replacement is inconvenient because the apparatus is complicated. When elements such as an image bearing member, a charging device, a developing device are integrally combined as a process cartridge as described in the embodiment of the present invention mentioned above, a user can easily replace the process cartridge and a small sized color image forming apparatus can be provided. 
   Next, another embodiment (i.e., a laser printer) of the image forming apparatus will be explained referring to drawings. 
     FIG. 10  is a schematic view illustrating a laser printer (hereinafter referred to as a printer) which is an embodiment of the image forming apparatus of the present invention. The printer is a printer for producing monochrome images, and includes a photoreceptor drum  20  serving as an image bearing member (i.e., a member to be charged). The photoreceptor drum  20  has a diameter of 30 mm, and is rotated by a driving device (not shown) in a direction indicated by an arrow A. The surface (i.e., the surface to be charged) of the photoreceptor drum  20  moves at a speed of 200 mm/sec. The surface of the photoreceptor drum  20  is uniformly charged with a charging device  40 . The distance between the surface of the photoreceptor drum  20  and exit openings of the charging device facing the photoreceptor drum is 1 mm. The charging device  40  discharges negative ions from the exit opening to adhere the negative ions to the surface of the photoreceptor drum  20 . Thus, the surface of the photoreceptor drum  20  is charged. The detail of the charging device  40  will be explained later in detail. 
   After the photoreceptor drum  20  is charged, an optical image writing unit (not shown) irradiates the charged surface of the photoreceptor drum  20  with light (L) including image information to form a latent electrostatic image. The thus prepared latent electrostatic image is developed with a developing device  32  (serving as developing means) using a developer including a toner. Thus, a toner image is formed on the photoreceptor drum  20 . Since the photoreceptor drum  20  is rotated, the toner image thereon is moved to a transfer region at which the photoreceptor drum  20  faces a transfer roller  33  serving as transfer means. On the other hand, a transfer paper P serving as a recording material, which has been fed from a paper feeding cassette (not shown) and stopped at a pair of registration rollers (not shown), is timely fed by the pair of registration rollers to the transfer region. The transfer paper P is fed through the transfer region by a paper feeding belt  34  (serving as recording material feeding means) while borne on the paper feeding belt  34 . A predetermined transfer bias is applied to the transfer roller  34 , and thereby a transfer current flows through the transfer region. Due to the thus applied transfer bias, the toner image on the photoreceptor drum  20  is transferred onto the transfer paper P at the transfer region. The transfer paper P bearing the toner image thereon is then fed to a fixing device  35  (serving as fixing means). The toner image is fixed on the transfer paper P by the fixing device  35  upon application of heat and pressure thereto. Then the transfer paper P bearing the fixed toner image thereon is discharged from the printer. 
   Toner particles remaining on the surface of the photoreceptor drum  20  after the transfer operation are removed therefrom with a cleaning unit  36  (serving as cleaning means). In addition, charges remaining on the surface of the photoreceptor drum  20  after the transfer operation are removed with a discharging lamp  37  (serving as discharging means). Although the printer of this embodiment uses the cleaning unit  36  to remove residual toner particles, the printer can have a cleaner-less configuration such that residual toner particles are collected with the developing device  32 , etc. 
   Next, the charging device  40  of the present invention will be explained. 
     FIG. 9  is a schematic view illustrating a cross section of an example of the charging device of the present invention, which can be preferably used for the printer mentioned above. 
   Referring to  FIG. 9 , a charging device  50  includes electrodes  21   a  and  21   b  (hereinafter referred to as electron discharging electrodes) which are located on both ends of the charging device in the direction A (i.e., the rotation direction of the photoreceptor drum  20 ) and which are made of a material such as aluminum. Electron discharging layers  22   a  and  22   b  are provided on the inner walls of the electron discharging electrodes  21   a  and  21   b , respectively. Each of the electron discharging layers  22   a  and  22   b  is made of a film of an electron discharging element or a layer in which a powder of an electron discharging element is fixedly dispersed. 
   In addition, the charging device  50  further includes an opposing electrode  23  which is located in the center of the charging device  50  and which is made of a material such as aluminum. These electrodes  21   a ,  21   b  and  23  are fixed on an insulating support  24  which is made of a material such as resins such that the distance between the electrode  21   a  or  21   b  and the opposing electrode  23  be a predetermined distance. A space B (hereinafter sometimes referred to as an ion generation space) formed between the electrode  21   a  or  21   b  and the opposing electrode  23  opens toward the photoreceptor drum  20  while the opposite side of the space B is shut by the insulating support  24 . Thus, exit openings  25   a  and  25   b  are formed at the lower end of the charging device  50 . 
   In this embodiment, a voltage is applied to the electrode discharging electrodes  21   a  and  21   b  from a DC power supply (not shown) serving as voltage application means. Therefore, an electric field is formed between the electrode  21   a  or  21   b  and the opposing electrode  23  which is grounded. Thereby electrons are discharged from the electron discharging layers  22   a  and  22   b . The thus discharged electrons are adhered to molecules of gaseous materials such as oxygen, carbon dioxide, and nitrogen gasses or combinations of such gaseous materials and water, thereby negatively ionizing the molecules, resulting in formation of negative ions. The thus formed negative ions escape from the exit openings  25   a  and  25   b , and adhere to the surface of the photoreceptor drum  20 . Therefore, the surface of the photoreceptor drum  20  are negatively charged. 
   The charging device  50  of the present invention has an advantage such that the amount of ozone and NOx generated in the charging operation is much lower than that in the case using a conventional charging device. The reason therefore is as follows. 
   In general, charging utilizing corona discharging generates a large amount of ozone and NOx. This is because electrons generated by corona discharging collide against molecules of gaseous materials, resulting in formation of ions. Specifically, electrons discharged from a corona wire have an energy of not less than 30 eV. Since the energy for ionizing a nitrogen molecule and an oxygen molecule is 24.3 eV, and 8 eV, respectively, electrons discharged from a corona wire can easily ionize molecules of nitrogen and oxygen. 
   In contrast, in the charging device  50  of the present embodiment, a relatively low voltage is applied to the electrodes  21   a  and  21   b  so as not to cause discharging in the space B, i.e., the voltage is lower than the threshold voltage in the Paschen&#39;s law over which discharging is caused. In addition, since the electron discharging element includes a 5H-BN material having sp 3  bond, electrons discharged from the element have an energy of about 6 eV. Therefore, ionization of nitrogen and oxygen is not caused. Namely, in the present charging device, ions can be formed without generating discharge-induced products such as ozone and NOx. 
   The 5H-BN materials having sp 3  bond have the same bond-structure as that of diamond, and are one kind of boron nitride compounds which have the hardness next to diamond. Boron nitride compounds have a good combination of heat resistance and chemical resistance so as to be used for crucibles. Therefore, the materials have excellent durability so as to be used as heavy duty electron discharging elements. 
   The 5H-BN materials having sp 3  bond can be prepared by irradiating a substrate made of a material such as silicon and nickel with a mixture gas plasma (diborane (i.e., hydrogenated boron (B 2 H 6 )), hydrogen, ammonia, and argon) and a defocused ultraviolet excimer laser having a wavelength of 193 nm and a frequency of 1030 Hz. As a result, a thin layer is formed on the substrate. The thin layer has a number of projections with a length of about 10 μm, which have a spindle form having a sharp tip and which are arranged so as to be parallel to the laser irradiation direction. The projections serve as electron emitters. 
   The electron discharging layers  22   a  and  22   b  are formed on the electron discharging electrodes  21   a  and  21   b , respectively, by forming a thin layer of a 5H-BN material having sp 3  bond or a layer in which a powder of a 5H-BN material is fixed using a binder such as electroconductive resins. Specific examples of the film forming methods include printing methods and sputtering methods. It is preferable for the electron discharging layers  22   a  and  22   b  that a number of particles of a 5H-BN material are present on the surface of the layers. 
   In this embodiment, a thin layer of a 5H-BN material or a layer in which a 5H-BN material is dispersed in a binder is adhered to the electrodes  21   a  and  21   b  using an electroconductive adhesive. Then the electrodes  21   a  and  21   b  bearing the respective electron discharging layers  22   a  and  22   b  are arranged so as to face the opposing electrode  23  with a gap therebetween. When a voltage is applied to the electrodes  21   a  and  21   b , electrons are discharged by the electron discharging layers  22   a  and  22   b , resulting in formation of negative ions in the spaces B. 
   In this embodiment, 5H-BN materials having sp 3  bond are used as the electron discharging material in view of safety and life. However, the electron discharging material is not limited thereto, and other electron discharging materials such as carbon nano tube, and diamond like carbon can also be used therefor. 
   In order to discharge electrons sufficient for charging the photoreceptor drum  20  so as to have a predetermined potential, the electron discharging layers  22   a  and  22   b  preferably has a large surface area. If the electron discharging layers  22   a  and  22   b  are set so as to face the photoreceptor drum  20  (similarly to the conventional charging devices), the charging device has to have a large width in the direction (not less than twice the vertical length of the layer  22   a  (or  22   b ) in  FIG. 9 ). 
   In contrast, in the present example, the electron discharging layers  22   a  and  22   b  are set so as to be parallel to the normal direction of the photoreceptor drum  20  as illustrated in  FIG. 9 . In this regard, the width of the charging device  50  depends on the gap between the electron discharging electrodes  21   a  and  21   b  and the opposing electrode  23 . In other words, the width of the charging device  50  does not depend on the vertical length of the electron discharging layers  22   a  and  22   b . Since the gap between the electron discharging electrodes  21   a  and  21   b  and the opposing electrode  23  is on the order of 300 μm, the length of the charging device  50  in the direction A can be dramatically decreased, resulting in miniaturization of the image forming unit of the image forming apparatus. Specifically, the length of the charging device  50  in the direction A is about 1.5 mm in this example. 
   Next, the structure of the electron discharging layers  22  ( 22   a  and  22   b ) will be explained in detail referring to  FIGS. 11-14 . In  FIGS. 11-13 , only the electron discharging layer  22   a  is illustrated because the other electron discharging layer  22   b  has the same structure. 
     FIG. 11  is an elevation view of the electron discharging layer  22   a  formed on the electron discharging electrode  21   a.    
   As illustrated in  FIG. 11 , the electron discharging electrode  21   a  has a rectangular form in which a length L 1  of the side of the rectangle parallel to the axial direction of the photoreceptor drum  20  is longer than the other side (i.e., the height thereof). The length L 1  is longer than a length L 3  of the image forming area within which an image is formed on the photoreceptor drum  20 . As illustrated in  FIG. 11 , the electron discharging electrode  21   a  and the electron discharging layer  22   a  are set such that the longitudinal direction thereof is parallel to the axial direction of the photoreceptor drum  20  and surfaces of the electrode and the layer are parallel to the normal direction of the photoreceptor drum  20 . Character L 2  denotes a length of the electron discharging layer  22   a.    
   In this embodiment, a voltage of −1200V is applied to the electron discharging electrode  21   a . As a result, the surface of the photoreceptor drum  20 , whose surface is rotated at a linear speed of 200 mm/s, is evenly charged to have a potential of −400V. 
     FIG. 12  is an elevation view of another example of the electron discharging layer  22   a  formed on the electron discharging electrode  21   a . The electron discharging layer  22   a  has two rectangular electron discharging portions in which the side thereof parallel to the axial direction of the photoreceptor drum  20  is longer than the other side thereof. Similarly to the electron discharging layer illustrated in  FIG. 11 , the length of the side of the electron discharging portion parallel to the axial direction of the photoreceptor drum  20  is longer than the length (L 3 ) of the image forming area of the photoreceptor drum  20 . In addition, the electron discharging electrode  21   a  and the electron discharging layer  22   a  are set similarly to the electron discharging electrode  11   a  illustrated in  FIG. 11 . As a result of the present inventors&#39; experiment, it is found that the electron discharging layer of this example has the same chargeability as that of the electron discharging layer illustrated in  FIG. 11 . 
   Next, another example of the electron discharging layer  12  will be explained. 
     FIG. 13  is an elevation view of another example of the electron discharging layer  22   a  formed on the electron discharging electrode  21   a.    
   The electron discharging layer  22   a  includes two lines of electron discharging portions each of which includes a plurality of small rectangular electron discharging portions arranged in the longitudinal direction of the electrodes  21   a  (i.e., in the axial direction of the photoreceptor drum  20 ). Similarly to the above-mentioned two examples of the electron discharging layer, the length of one line of the electron discharging portion in the longitudinal direction is longer than the length (L 3 ) of the image forming area of the photoreceptor drum  20 . In addition, the electron discharging electrode and the electron discharging layer are set similarly to the electron discharging electrode  21   a  illustrated in  FIG. 11 . As illustrated in  FIG. 13 , the electron discharging portions in one line are arranged to face the gaps between the electron discharging portions in another line. By taking such a configuration, the electron discharging portions can evenly discharge electrons in the longitudinal direction thereof. As a result of the present inventors&#39; experiment, it is found that the electron discharging layer of this example has the same chargeability as that of the electron discharging layers illustrated in  FIGS. 11 and 12 . 
   In the above-mentioned three examples of the electron discharging layer, both the electron discharging layers  22   a  and  22   b  have the same structure. However, the electron discharging layers  22   a  and  22   b  can have different structures. For example, it is possible to use the electron discharging layers illustrated in  FIGS. 11 and 12  for the electron discharging layers  22   a  and  22   b , respectively. 
   Next, another example of the electron discharging layer  22  will be explained. 
     FIG. 14  is an elevation view of another example of the electron discharging layers  22   a  and  22   b  formed on the electron discharging electrode  21   a  and  21   b , respectively. 
   The electron discharging layer  22   a  includes a line of electron discharging portions including a plurality of small rectangular electron discharging portions arranged in the longitudinal direction of the electrodes  21   a  (i.e., in the axial direction of the photoreceptor drum  20 ). Similarly, the electron discharging layer  22   b  includes a line of electron discharging portions including a plurality of small rectangular electron discharging portions arranged in the longitudinal direction of the electrodes  21   b  (i.e., in the axial direction of the photoreceptor drum  20 ). Similarly to the above-mentioned three examples of the electron discharging layer, the length of the line of electron discharging portions in the longitudinal direction is longer than the length of the image forming area of the photoreceptor drum  20 . In addition, the electron discharging electrode  21   a  and the line of electron discharging portions are set similarly to the electron discharging electrode  21   a  illustrated in  FIG. 11 . 
   As illustrated in  FIG. 14 , the electron discharging portions of the electron discharging layer  22   b  formed on the electron discharging electrode  21   b  are set so as to face the gaps formed between the electron discharging portions of the electron discharging layer  22   a  formed on the electron discharging electrode  21   a . By taking such a configuration, the electron discharging layers  22   a  and  22   b  can evenly discharge electrons in the longitudinal direction thereof. As a result of the present inventor&#39;s experiment, it is found that the electron discharging layer of this example has the same chargeability as that of the electron discharging layers illustrated in  FIGS. 11 to 13 . 
   Next, another example of the charging device of the present invention will be explained. 
     FIG. 15  illustrates the cross section of another example of the charging device. As can be understood from  FIG. 15 , a charging device  110  has the same configuration as that of the charging device illustrated in  FIG. 9  except that electron discharging layers  112   a  and  112   b  are formed on both sides of a center electrode  111  serving as an electron discharging electrode. Electrodes  113   a  and  113   b  serves as opposing electrodes. In addition, the charging device  110  includes a grid electrode  116  configured to transport negative ions formed in the spaces B toward exit openings  115   a  and  115   b , i.e., toward the surface of the photoreceptor drum  20 . 
   The electron discharging layers  112   a  and  112   b  formed on the electron discharging electrode  111  have a structure similar to those illustrated in  FIGS. 10 to 14 . 
   When a first DC power source  117  applies a voltage to the electron discharging electrode  111  and a second DC power source  118  applies a voltage to the opposing electrodes  113   a  and  113   b , an electric field is formed between the electron discharging electrode  111  and the opposing electrodes  113   a  and  113   b , and thereby negative ions are formed in the spaces B. 
   In this example, the grid electrode  116  is made of stainless steel, and a voltage is applied to the grid electrode  116  by a grid power source  119 . Similarly to the grid electrodes used for conventional scorotron charging devices, the grid electrode  116  has a honey comb form. However, the grid electrode  116  is not limited thereto, and any grid electrodes which can pass ions can be used therefor. For example, electroconductive films and electroconductive plate having openings can also be used. The distance between the tip of the electron discharging electrode  111  and the grid electrode  116  and the distance between the tips of the opposing electrodes  113   a  and  113   b  and the grid electrode  116  are set at a distance such that discharging is not caused. Specifically, in this embodiment, the distance is set to be 500 μm. In addition, the distance between the grid electrode  116  and the surface of the photoreceptor drum  20  is set to be 1 mm. 
   The negative ions generated in the space B are transported toward the surface of the photoreceptor drum  20  by the grid electrode  116  to which a voltage is applied. In order to generate negative ions and transport the ions toward the photoreceptor drum, it is preferable to satisfy the following relationship (1):
 
| A|&gt;|B|&gt;C   (1)
 
wherein A represents the voltage (V) applied to the electron discharging electrode  111 ; B represents the voltage (V) applied to the opposing electrodes  113   a  and  113   b ; and C represents the voltage (V) applied to the grid electrode  116 .
 
   When the relationship (1) is satisfied, the electrons discharged from the electron discharging layers  112   a  and  112   b  move toward the opposing electrodes  113   a  and  113   b . When the electrons collide against molecules of gaseous materials in the space B, negative ions are formed. The thus formed negative ions move toward the grid electrode  116 . Since the potential of the surface of the photoreceptor drum  20  has been decreased to about 0V by the discharging lamp  37 , the negative ions pass the grid electrode  116  and adhere to the surface of the photoreceptor drum  20 . After the potential of the surface of the photoreceptor drum  20  is increased to substantially the same voltage as that of the grid electrode  116  due to adhesion of negative ions, negative ions present in the vicinity of the grid electrode  116  do not move toward the photoreceptor drum  20 . Thus, transportation of negative ions can be accelerated by the migration electric field formed by the voltage applied to the grid electrode  116 . Numeral  114  denotes an insulating support. 
   In this example, a voltage of −1200V is applied to the electron discharging electrode  111  by the first DC power source  117 ; a voltage of −800V is applied to the opposing electrodes  113   a  and  113   b  by the second DC power source  118 ; and a voltage of −650V is applied to the grid electrode  116  by the grid power source  119 . As a result, the surface of the photoreceptor drum  20  is evenly charged to a potential of −600V, which is much higher than the potential (−400V) of the photoreceptor drum  20  in the examples mentioned above. Thus, it is confirmed that the grid electrode  116  can enhance the charging ability of the charging device. In  FIG. 15 , character GND means a ground. 
     FIG. 16  is a cross section of another example of the charging device of the present invention. 
   As can be understood from  FIG. 16 , a charging device  250  has the same configuration as that of the charging device  110  mentioned above except that four ion generation spaces B are formed. Specifically, the charging device  250  has three opposing electrodes  253   a ,  253   b  and  253   c , and two electron discharging electrodes  251   a  and  251   b , each of which is located between two opposing electrodes. On both sides of the electron discharging electrodes  251   a  and  251   b , electron discharging layers  252   a ,  252   b ,  252   c  and  252   d  are formed. The voltages applied to the electrodes are the same as those in the charging device  110  in  FIG. 15 . 
   In this charging device  250 , the total surface area of the electron discharging layer is twice that in the charging device  110 . Therefore, the amount of electrons generated by the electron discharging layers can be dramatically increased, and thereby the amount of generated negative ions can be dramatically increased. Therefore, this charging device can be preferably used for high speed image forming apparatus. Numerals  254  and  256  denote an insulating support, and a grid electrode. 
     FIG. 17  is a cross section of another example of the charging device of the present invention. 
   As can be understood from  FIG. 17 , a charging device  330  has the same configuration as that of the charging device  110  mentioned above except that an insulating support  334  has openings  340   a  and  340   b  configured to supply outside air to the spaces B. The charging device  330  includes an electron discharging electrode  331 , opposing electrodes  333   a  and  333   b  and the insulating support  334 , which surround the ion generation spaces B. In addition, the charging device  330  has exit openings  335   a  and  335   b  through which generated ions move toward the surface of the photoreceptor drum; and the openings  340   a  and  340   b  through which outside air is supplied to the spaces B. 
   Since the charging device  330  has the openings  340   a  and  340   b  on the upper portion of the spaces B, air can be smoothly flown, and thereby negative ions can be efficiently discharged from the exit openings  335   a  and  335   b . In this regard, air is flown from the openings  340   a  and  340   b  to the openings  335   a  and  335   b . Therefore, the negative ions are further efficiently discharged from the openings  335   a  and  335   b . Thus, negative ions can be efficiently transported to the surface of the photoreceptor drum, resulting in enhancement of the charging ability of the charging device. 
   It is preferable to arrange air supplying openings (i.e., the openings  340   a  and  340   b ) so as to face the openings  335   a  and  335   b , respectively, because air can be linearly flown through the spaces B, thereby efficiently discharging negative ions from the exit openings  335   a  and  335   b . Numeral  336  denotes a grid electrode. 
   Air supplying openings can be preferably used for not only a charging device having a grid electrode but also a charging device including no grid electrode. 
   In this example, air supplying openings are formed so as to face the exit openings  335   a  and  335   b . However, air supplying openings can be provided at such a location as not to face the exit openings  335   a  and  335   b . For example, in a charging device  350  illustrated in  FIG. 18 , opposing electrodes  353   a  and  353   b  have respective air supplying openings  360   a  and  360   b  on an upper portion thereof. In this case, the openings  360   a  and  360   b  are preferably formed at locations so as to be apart from the openings  335   a  and  335   b  as far as possible. In addition, it is preferable that the openings  360   a  and  360   b  do not face the electron discharging layers  332   a  and  332   b . In this case, an electron discharging electrode  351  may have an opening (i.e., a passage)  361  to connect the two spaces B with each other. 
   In this example, each of the air supplying openings  360   a  and  360   b  is one opening which fully extends in the longitudinal direction of the charging device (i.e., in the axial direction of the photoreceptor drum  20 ). However, the openings are not limited thereto, and one partial opening or combinations of plural openings can also be used therefore. 
   In addition, the charging device can include a fan (i.e., air flow generating means) configured to forcibly supply air from the openings  360   a  and  360   b  to the spaces B. In this case, the air in the spaces B can be flown at a high speed toward the exit openings  335   a  and  335   b , thereby efficiently discharging negative ions from the exit openings  335   a  and  335   b . A fan, which is provided in the printer for another purpose, can be used for the air flow generating means instead of the fan provided in the charging device. Numerals  351  and  354  denote an electron discharging electrode and an insulating support, respectively. 
     FIG. 19  is a cross section of another example of the charging device of the present invention. 
   In a charging device  410  illustrated in  FIG. 19 , electron discharging layers  412   a  and  412   b  formed on an electron discharging electrode  411  have the configuration as illustrated in  FIG. 12 . In addition, opposing electrodes  413   a  and  413   b  are formed on an insulating support  422   a  so as to face the electron discharging layer  412   a , and opposing electrodes  413   c  and  413   d  are formed on an insulating support  422   b  so as to face the electron discharging layer  412   b . The opposing electrodes  413   a  and  413   c  are farther apart from openings  415   a  and  415   b  than the opposing electrodes  413   b  and  413   d.    
   In each of the ion generating spaces, two electric fields are formed by an upper electric field forming section (i.e., a combination of upper one of the electron discharging layer  412   a  (or  412   b ) and the opposing electrode  413   a  (or  413   c )) and a lower electric field forming section (i.e., a combination of lower one of the electron discharging layer  412   a  (or  412   b ) and the opposing electrode  413   b  ( 413   d )). 
   In the charging device  410 , a voltage is applied from the second DC power source  118  to the opposing electrodes  413   a - 413   d  while controlled by a controller  423 . Specifically, the controller  423  controls the voltage applied to the opposing electrodes such that at first an electric field is formed in the lower electric field forming section and then an electric field is formed in the upper electric field forming section. 
   Specifically, at first the controller  423  applies a voltage of −800V to the opposing electrodes  413   b  and  413   d  to form an electric field in each of the lower electric field forming sections while applying a voltage of −1200V to the opposing electrodes  413   a  and  413   c  not to form an electric field in each of the upper electric field forming sections. In this regard, a voltage of −1200V is applied to the electron discharging electrode  411 . Thus, negative ions are formed in the lower portions of the spaces B near exit openings  415   a  and  415   b  while ions are not formed in the upper portions of the ion generation spaces. The thus generated negative ions in the lower portions of the ion generation spaces are efficiently moved toward the surface of the photoreceptor drum  20  due to the migration electric field of a grid electrode  416  and the voltage applied to the upper opposing electrodes  413   a  and  413   c.    
   The controller  423  then applies a voltage of −800V to the opposing electrodes  413   a  and  413   c  to form an electric field in each of the upper electric field forming sections while applying a voltage of −1200V to the opposing electrodes  413   b  and  413   d  not to form an electric field in each of the lower electric field forming sections. Thus, negative ions are formed in the upper portions of the ion generation spaces while ions are not formed in the lower portions of the ion generation spaces. Since a voltage of −1200V is applied to the opposing electrodes  413   b  and  413   d , the negative ions formed in the upper electric field forming sections cannot move toward the lower electric field forming sections and remain in the upper electric field forming sections. 
   The controller  423  then applies a voltage of −800V to the opposing electrodes  413   b  and  413   d  to form an electric field in each of the lower electric field forming sections while applying a voltage of −1200V to the opposing electrodes  413   a  and  413   c  not to form an electric field in each of the upper electric field forming sections. In this case, negative ions are formed in the lower electric field forming sections, and in addition the negative ions remaining in the upper electric field forming sections move toward the lower electric field forming sections. Thus, these negative ions move toward the surface of the photoreceptor drum  20 . 
   By repeating these voltage application operations, negative ions generated in the upper electric field forming sections are sequentially transported to the surface of the photoreceptor drum  20 . Namely, by using this charging device, movement of negative ions generated in the upper portions of the ion generation spaces can be accelerated. When such a voltage control operation is not performed, the negative ions generated in the upper portions of the ion generation spaces tend to be attracted by the opposing electrodes even if a grid electrode is provided. Therefore, it is preferable to perform such a voltage control operation in order to efficiently move the negative ions in the ion generation spaces toward the surface of the photoreceptor drum  20 . 
   In particular, by timely changing the voltage applied to the lower opposing electrodes and the upper opposing electrodes, negative ions can be efficiently transported toward the surface of the photoreceptor drum  20  at a high speed. Therefore, this charging device has a higher charging ability. 
   In this example, the electron discharging layers  412   a  and  412   b  have the configuration as illustrated in  FIG. 12 . However, the configuration of the electron discharging layers  412   a  and  412   b  is not limited thereto, and other configurations such as those illustrated in  FIGS. 11 ,  13  and  14  can also be available. One example is a charging device  450  illustrated in  FIG. 20 , in which electron discharging layers  452   a  and  452   b  have the configuration as illustrated in  FIG. 11 . 
     FIG. 21  is a cross section of another example of the charging device of the present invention. 
   In a charging device  510  illustrated in  FIG. 21 , the voltages applied to electron discharging electrodes  511   a ,  511   b ,  511   c  and  511   d  are controlled. Specifically, the charging device  510  includes opposing electrodes  513   a  and  513   b  which are located on both sides of the charging device; and an insulating electrode support  522  which is located in the center of the charging device. On one side of the insulating electrode support  522 , the electron discharging electrodes  511   a  and  511   b  are formed side by side in the vertical direction. Similarly, on the other side of the insulating electrode support  522 , the electron discharging electrodes  511   c  and  511   d  are formed side by side in the vertical direction. In addition, electron discharging layers  512   a ,  512   b ,  512   c  and  512   d  are formed on the electron discharging electrodes  511   a ,  511   b ,  511   c  and  511   d , respectively. 
   As can be understood from  FIG. 21 , the electron discharging layers  511   a  and  511   c  are farther apart from exit opening  515   a  and  515   b  than the electron discharging layers  511   b  and  511   d . Thus, the charging device  510  has upper electric field forming sections in which an electric field is formed by a combination of the electron discharging layer  511   a  and the opposing electrode  513   a  and a combination of the electron discharging layer  511   c  and the opposing electrode  513   b , and lower electric field forming sections in which an electric field is formed by a combination of the electron discharging layer  511   b  and the opposing electrode  513   a  and a combination of the electron discharging layer  511   d  and the opposing electrode  513   b.    
   By performing a voltage controlling similar to that mentioned above in the charging device illustrated in  FIG. 19 , negative ions generated in the upper portions of the ion generation spaces can be efficiently transported toward the surface of the photoreceptor drum  20 . Numeral  516  denotes a grid electrode. 
     FIG. 22  is a cross section of another example of the charging device of the present invention. 
   In a charging device  610  illustrated in  FIG. 22 , second opposing electrodes  624   a  and  624   b  are formed on an insulating support  614 . A voltage (e.g., −800V) higher than the voltage (−650V) applied to a grid electrode  616  is applied to the second opposing electrodes  624   a  and  624   b . By applying such a voltage to the second opposing electrodes  624   a  and  624   b , an electric field is formed in spaces Ba and Bb, and thereby negative ions in the spaces Ba and Bb can be efficiently transported toward openings  615   a  and  615   b.    
   It is possible for the charging device  610  to control the voltage applied to an electron discharging electrode  611  and at least one of the voltages applied to opposing electrodes  613   a  and  613   b  to alternately form an electric field in the space Ba and the space Bb. In this case, a voltage is preferably applied to the second opposing electrodes  624   a  (or  624   b ) when an electric field is not formed in the space Ba (or Bb). By thus controlling application of voltage, an electric field for moving negative ions in the space Ba (or Bb), in which no other electric field is formed, toward the grid electrode  616  is formed. Therefore, the negative ions in the space Ba (or Bb) can be efficiently transported to the surface of the photoreceptor drum  20 . Numerals  612   a  and  612   b  denote electron discharging layers. 
     FIG. 23  is a plan view of another example of the charging device of the present invention. 
   A charging device  710  illustrated in  FIG. 23  includes a spacer  725  on both sides of the charging device to keep the distance between an electron discharging electrode  711  and an opposing electrode  713   a  or  713   b  constant. Specifically, the spacer includes an insert portion  725   a , and the insert portion  725   a  is inserted into gaps between the electron discharging electrode  711  and the opposing electrodes  713   a  and  713   b . By providing such a spacer, the distance between an electron discharging electrode  711  and the opposing electrode  713   a  or  713   b  can be precisely controlled, and thereby a uniform electric field can be stably formed in the ion generation spaces therebetween. Therefore, a constant amount of negative ions can be formed in the spaces, and thereby the photoreceptor drum can be uniformly charged. In  FIG. 23 , numerals  712   a  and  712   b  denote an electron discharging layer. 
     FIG. 24  is a cross section of another example of the charging device of the present invention. 
   In a charging device  810  illustrated in  FIG. 24 , a grid electrode  816  also serves as an electrode supporting mechanism configured to keep the distance between an electron discharging electrode  811  and an opposing electrode  813   a  or  813   b  constant. Specifically, the electrode supporting mechanism includes the grid electrode  816  and a fixing member  816   a  which is made of an insulating material and which fixes the electron discharging electrode  811  and opposing electrodes  813   a  and  813   b  to the grid electrode  816  such that the distances between the electron discharging electrode  811  and opposing electrodes  813   a  and  813   b  can be stably kept constant. Therefore, a uniform electric field can be stably formed in the ion generation spaces therebetween. Accordingly, a constant amount of negative ions can be formed in the ion generation spaces, and thereby the photoreceptor drum can be uniformly charged. In  FIG. 24 , numerals  812   a  and  812   b  denote an electron discharging layer, and numeral  814  denotes an insulating support. 
   The above-mentioned charging devices serve as charge applying devices for charging the photoreceptor drum  20 . Negative ions generated in the ion generation spaces in the discharging devices are adhered to the surface of the photoreceptor drum  20 . Thus, the image forming portion of the photoreceptor drum  20  is charged. In conventional charging devices used for charging a photoreceptor, the photoreceptor serves as an opposing electrode. In contrast, an opposing electrode, which is not a photoreceptor, is provided as a part in the charging device of the present invention. Therefore, the distance between the electrodes can be stably kept constant. Therefore, a predetermined electric field can be stably formed in the charging device, and thereby the amount of electrons discharged from the electron discharging layers can be controlled to be constant. Accordingly, the photoreceptor drum  20  can be stably charged so as to have a predetermined potential. 
   In addition, since the photoreceptor drum  20  does not serve as an opposing electrode in the charging device of the present invention, problems such as deterioration (oxidation and destruction) of the materials used for the photoreceptor drum (such as binder resins (e.g., polycarbonate resins) and photosensitive materials) are not caused, and thereby abrasion of the photosensitive layer can be avoided and the life of the photoreceptor drum can be prolonged. Further, discharge-induced materials such as ozone and NOx are hardly produced. Furthermore, in the charging device of the present invention a relatively low voltage is applied to the charging members compared to that applied to charging members in conventional charging devices. Therefore, energy can be saved. 
   In the charging devices mentioned above, the electron discharging electrodes and the opposing electrodes are plates and are provided so as to parallel to each other, and the electron discharging member is an electron discharging layer which is formed on the electron discharging electrodes and which has substantially a uniform thickness. Therefore, the amount of electrons discharged from the electron discharging member can be controlled so as to be constant, and thereby the surface of the photoreceptor drum  20  can be uniformly charged so as to have a predetermined potential. 
   In the charging devices illustrated in  FIGS. 11 and 12 , the electron discharging layers extend in the axial direction of the photoreceptor drum  20 . Therefore, the surface area of the electron discharging layers can be maximized in a limited space. Therefore, the charging devices can produce a large amount of electrons. 
   In the charging devices illustrated in  FIGS. 13 and 14 , a plurality of electron discharging portions serving as an electron discharging layer are arranged in the longitudinal direction of the charging devices. When a long electron discharging layer (such as the layers illustrated in  FIGS. 11 and 12 ) is prepared, there is a case where the resultant electron discharging layer has unevenness particularly in the longitudinal direction thereof, resulting in uneven charging of the photoreceptor drum in the axial direction of the photoreceptor drum. By using an electron discharging layer including a plurality of electron discharging portions, such uneven charging can be avoided even when discharging ability of the electron discharging portions slightly varies. The reason therefor is that since such varied electron discharging members are randomly arranged, the variation tends to be cancelled and the resultant electron discharging layers tend to have an average electron discharging ability in the longitudinal direction thereof. Therefore, the amount of electrons discharged from the electron discharging member can be controlled so as to be constant, and thereby the surface of the photoreceptor drum  20  can be uniformly charged. 
   In particular, in the electron discharging layers illustrated in  FIGS. 13 and 14 , plural electron discharging portions are arranged in the longitudinal direction thereof without forming an electron non-discharging portion. Specifically, even though the electron discharging portions are arranged with a gap therebetween, the gap is arranged so as to face another electron discharging portion. Therefore, the photoreceptor drum  20  can be uniformly charged in the axial direction thereof. 
   In the charging device illustrated in  FIG. 19 , a plurality of electron discharging layers are arranged in the vertical direction to form upper and lower electric field forming sections. By forming an electric field in the lower electric field forming section, followed by formation of an electric field in the upper electric filed forming section, the negative ions generated in the ion generation spaces can be efficiently transported to the exit openings  415   a  and  415   b . Therefore, the image forming surface of the photoreceptor drum  20  can be uniformly charged. 
   In the charging devices illustrated in  FIGS. 15-24 , a grid electrode is provided in the vicinity of the exit openings of the spaces B to move the negative ions toward the image forming surface of the photoreceptor drum  20 . In the charging devices, the following relationship (1) is satisfied:
 
| A|&gt;|B|&gt;|C|   (1)
 
wherein A represents the voltage (V) applied to the electron discharging electrode; B represents the voltage (V) applied to the opposing electrode; and C represents the voltage (V) applied to the grid electrode.
 
   In this case, the negative ions generated in the spaces can be efficiently transported toward the photoreceptor drum and thereby the image forming surface of the photoreceptor drum can be efficiently charged. 
   The charging devices illustrated in  FIGS. 17 and 18  include air supplying openings which can form air flow to supply air to the spaces B. As a result, the negative ions generated in the spaces B can be efficiently transported to the photoreceptor drum. Therefore, the charging devices have an improved charging ability. In addition, since flesh air is supplied to the spaces B, ionization of gaseous materials in the spaces B can be stably performed, and thereby charging can be stably performed. 
   The charging devices illustrated in FIGS.  9  and  15 - 24  have an electrode supporting mechanism (e.g., the insulating supports and the spacer). Therefore, the distance between the electron discharging electrode and the opposing electrode can be kept constant, and thereby an electric field can be stably formed in the spaces B. Accordingly, the amount of electrons discharged from the electron discharging layers can be controlled so as to be substantially constant, and thereby a constant amount of negative ions can be generated in the spaces B. Therefore, the image forming surface of the photoreceptor drum can be stably charged. In particular, the charging device illustrated in  FIG. 23  includes a spacer as the electrode supporting mechanism. Therefore, the distance between the electron discharging electrode and the opposing electrode can be easily kept constant. 
   The charging device illustrated in  FIG. 24  has an electrode supporting mechanism which is constituted of the grid electrode and a fixing member configured to fix the electron discharging electrode and the opposing electrodes to the grid electrode. Therefore, the distance between the electron discharging electrode and the opposing electrodes can be easily controlled so as to be a predetermined distance. 
   The image forming apparatus (i.e., the printer) of the present invention includes an image forming device including a photoreceptor drum serving as an image bearing member, and one of the above-mentioned charging devices. The charging device is set in the printer such that the exit openings face the image forming surface of the photoreceptor drum (i.e., the electron discharging electrode and the opposing electrode are parallel to the normal direction of the photoreceptor drum). 
   Since the width of the charging device in the rotation direction of the photoreceptor drum depends on the distance between the electron discharging electrode and the opposing electrode, and does not depend on the surface area of the electron discharging layer. Therefore, the surface area of the electron discharging layer can be increased to enhance the charging ability of the charging device without increasing the width of the charging device in the rotation direction of the photoreceptor drum. Therefore, a miniaturized compact image forming device can be provided. 
   The image forming apparatus mentioned above is a monochrome printer. However, the image forming apparatus of the present invention is not limited thereto, and can be applied to multi-color (or full color) image forming apparatus. 
     FIG. 25  is a schematic view illustrating a tandem-type full color image forming apparatus according to the present invention. 
   Referring to  FIG. 25 , the image forming apparatus includes four image forming devices, each of which includes the photoreceptor drum  20 , the charger  40 , which is one of the charging devices mentioned above and which charges the photoreceptor drum, a developing device  52  configured to develop a latent electrostatic image with a developer including a toner to from a toner image on the photoreceptor drum  20 , and a cleaning device  56  configured to clean the surface of the photoreceptor drum after the transfer process. Light (L) including image information, which is emitted by an optical image writing device (not shown), irradiates the charged photoreceptor drum to form a latent electrostatic image on the photoreceptor drum  20 . 
   The color toner images (such as yellow, magenta, cyan and black color images) thus formed on the four photoreceptor drums  20  are then transferred to a recording material which is fed by a transfer belt  54 , resulting in formation of a full color toner image on the recording material. 
   The charging device of the present invention can be used as not only the above-mentioned charger configured to uniformly charge an image bearing member such as photoreceptors, but also a discharger configured to discharge charges remaining on an image bearing member after a toner image is transferred onto a recording material, and a transfer charger configured to charge a transfer member to clearly transfer a toner image on an image bearing member to a recording material. 
   In addition, the charging device can also be used for chargers for use in various apparatuses other than image forming apparatuses. 
   This document claims priority and contains subject matter related to Japanese Patent Applications Nos. 2005-120490 and 2005-254373, filed on Apr. 19, 2005, and Sep. 2, 2005, respectively, the entire contents of which are incorporated herein by reference. 
   Having now fully described the invention, it will be apparent to one of ordinary skill in the art that many changes and modifications can be made thereto without departing from the spirit and scope of the invention as set forth therein.