Abstract:
A heater includes: a hollow tube sealed hermetically and reduced in pressure, the interior of the hollow tube being divided by at least one partition wall into a plurality of regions arranged side by side in a longitudinal direction of the tube; an electron source provided in each of the divided regions, the electron source being supported inside the hollow tube in an electrically insulated state from the hollow tube; a power supply that applies a voltage between the hollow tube and the electron source with the electron source as a negative polarity side; and the electron source emitting electrons toward an inner periphery surface of the hollow tube to heat the hollow tube.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is based on and claims priority under 35 USC 119 from Japanese Patent Application No. 2006-104999 filed Apr. 6, 2006. 
       BACKGROUND 
       [0002]    1. Technical Field 
         [0003]    The present invention relates to a heater wherein voltage is applied between opposed electrodes in a pressure-reduced state and an anodic electrode (anode) is heated by electrons emitted from a cathodic electrode (cathode) as an electron source, a fixing device for fixing a toner image with use of the heater, and an image forming apparatus. 
         [0004]    2. Related Art 
         [0005]    Generally, as a toner image fixing process in an image forming apparatus using a powdered toner, there is widely adopted a process wherein a toner image is transferred electrostatically onto a recording medium, then the recording medium is sandwiched in between a heating member and a pressurizing member to heat the toner image and pressure-bond the toner image to the recording medium. 
         [0006]    In such a fixing device, a halogen lamp is in wide use as a heating source for heating the toner image. For example, a fixing roller having a halogen lamp in the interior of a cylindrical core and a pressing roller to be pressed against the fixing roller are provided and a nip portion is formed wherein rotating fixing roller and pressing roller are pressed against each other. A recording medium which carries an unfixed toner image thereon is fed to the nip portion and is conveyed. The fixing roller is heated by radiant heat radiated from the halogen lamp disposed in the interior, whereby the toner image on the recording medium passing through the nip portion is heated and pressurized. 
         [0007]    Recently, as a measure against environmental issues it has also been required to suppress the consumption of energy. More particularly, in order to make the aforesaid warming-up time as short as possible and minimize the power consumption in stand-by condition, it is required that a portion of a fixing member for heating a toner image be heated locally efficiently. 
         [0008]    In this connection, a fixing device which adopts an electromagnetic induction heating method instead of using a halogen lamp as a heating source is proposed. 
       SUMMARY 
       [0009]    According to an aspect of the invention, there is provided a heater including: a hollow tube sealed hermetically and reduced in pressure, the interior of the hollow tube being divided by at least one partition wall into a plurality of regions arranged side by side in a longitudinal direction of the tube; an electron source provided in each of the divided regions, the electron source being supported inside the hollow tube in an electrically insulated state from the hollow tube; a power supply that applies a voltage between the hollow tube and the electron source with the electron source as a negative polarity side; and the electron source emitting electrons toward an inner periphery surface of the hollow tube to heat the hollow tube. 
     
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]    Exemplary embodiments of the present invention will be described in detail based on the following figures, wherein: 
           [0011]      FIG. 1  is a schematic sectional view of a fixing device exemplarily embodying the present invention; 
           [0012]      FIG. 2  is a sectional view taken on line A-A of the fixing device shown in  FIG. 1 ; 
           [0013]      FIG. 3  is an exploded perspective view showing a pressing resistance member, a heater and a fixing belt guide member used in the fixing device shown in  FIG. 1 ; 
           [0014]      FIGS. 4A and 4B  are sectional views of a heater exemplarily embodying the present invention and used in the fixing device of  FIG. 1 ; 
           [0015]      FIG. 5  is an enlarged sectional view of an electron source used in the heater of  FIG. 4 ; and 
           [0016]      FIG. 6  is a schematic construction diagram showing an image forming apparatus using a fixing device according to an exemplary embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0017]    Exemplary embodiments of the present invention will be described hereunder with reference to the drawings. 
         [0018]      FIG. 6  is a schematic construction diagram showing an image forming apparatus using a fixing device according to an exemplary embodiment of the present invention. 
         [0019]    The image forming apparatus shown in  FIG. 6  is provided with a cylindrical photosensitive drum  41  on the surface of which is formed a latent image based on an electrostatic potential difference upon exposure to image light after uniform charging. The image forming apparatus is further provided around the photosensitive drum  41  with a charging device  42  for charging the surface of the photosensitive drum  41  uniformly, an exposure device  43  that radiates image light to the photosensitive drum  41  to form a latent image on the drum surface, a developing unit  44  that causes toners to be transferred selectively onto the latent image on the photosensitive drum to form a toner image, an endless belt-like intermediate transfer member  45  opposed to the photosensitive drum  41  and whose peripheral surface is supported so as to be movable circumferentially, a cleaning device  46  for removing toner remaining on the photosensitive drum after transfer of the toner image, and a destaticizer  47  for destaticizing the surface of the photosensitive drum  41 . 
         [0020]    Inside the intermediate transfer member  45  are disposed a transfer charger  48  for a first transfer of the toner image formed on the photosensitive drum onto the intermediate transfer member  45 , two support rollers  49   a  and  49   b,  and a transfer opposition roller  50  for performing a second transfer. With these components, the intermediate transfer member  45  is stretched bridgewise so as to be movable circumferentially. A transfer roller  51  for transfer of the toner image on the intermediate transfer member to recording paper is disposed at a position opposed to the transfer opposition roller  50  through the intermediate transfer member  45 . Recording paper P is fed from a paper tray (not shown) to a pressure contact portion between the transfer opposition roller  50  and the transfer roller  51 . Downstream of the pressure contact portion are disposed a fixing device  1  for heat-melting the toner image on the recording paper and bringing it into pressure contact with the recording paper and a cleaning device  53  for removing toner remaining on the intermediate transfer member, the cleaning device  53  being disposed at a position along the intermediate transfer member  45 . Further provided is a controller  60  for controlling various portions of the image forming apparatus. 
         [0021]      FIG. 1  is a schematic sectional view of a fixing device according to an exemplary embodiment of the present invention and  FIG. 2  is a sectional view taken on line A-A in  FIG. 1 . 
         [0022]    The fixing device includes a fixing belt  1  having an endless peripheral surface, a pressing roller  2  adapted to be pressed against the outer periphery surface of the fixing belt  1 , a heater  3  for heating the fixing belt  1 , the heater  3  being abutted against the inner periphery surface of the fixing belt  1  at a position where the pressing roller  2  is pressed, and a pressing resistance member  4  supported fixedly by the inside of the endless fixing belt, the pressing resistance member  4  supporting the heater  3  at a predetermined position and resisting to the pressing force of the pressing roller  2 . A recording sheet P which carries a toner image T is fed between the fixing belt  1  and the pressing roller  2  to heat and press the toner image T and fix it onto the recording sheet. A peeling member  9  is disposed downstream of the nip portion where the fixing belt  1  and the pressing roller  2  are brought into pressure contact with each other to peel and discharge the recording sheet from the fixing belt  1 . 
         [0023]    The fixing belt  1  is composed of a base layer formed by a thin film of stainless steel (SUS) and a surface release layer laminated onto the base layer. 
         [0024]    As the base layer there may be used, for example, a thin layer of SUS having a thickness of 40 to 80 μm. In this example, there is used a thin film of SUS having a thickness of 50 μm. 
         [0025]    The surface release layer is a layer which comes into direct contact with an unfixed toner image transferred onto the recording sheet. Therefore, polyimide resin or fluorine resin superior in releasability and durability may be used as the material of the surface release layer. In this example there is used a PFA layer having a thickness of 30 μm, provided it suffices for the layer to have a thickness of 1 to 30 μm. 
         [0026]    The inner periphery surface of the base layer may be coated with fluorine resin in order to diminish the sliding resistance for the heater  3  abutted against the inner periphery surface of the base layer. A release agent such as, for example, silicone oil may be applied as lubricant to the inner surface of the fixing belt  1 . 
         [0027]    On the other hand, between the base layer and the surface release layer, there may be provided an elastic layer superior in heat resistance and heat conductivity such as, for example, silicone rubber, fluorine-containing rubber, or fluorosilocone rubber. 
         [0028]    The pressing roller  2  is supported at a position opposed to the fixing belt  1  and both end portions thereof are urged toward the fixing belt  1  by means of springs  5 . The pressing roller  2  includes a metallic cylindrical member  2   a  as a core member, an elastic member  2   b  having heat resistance such as silicone rubber or fluorine-containing rubber and formed on the surface of the cylindrical member  2   a,  and a surface release layer (not shown) as an outermost surface layer. The pressing roller  2  is rotated by a motor  10  and the fixing belt  1  is driven by friction induced in the portion where the fixing belt  1  is brought into pressure contact with the pressing roller  2 . In this example, the pressing roller  2  is urged toward the heater  3  through the fixing belt  1  at a total load of 294 N (30 kgf). 
         [0029]    As shown in.  FIG. 3 , the pressing resistance member  4  is a rod-like member having an axis in the width direction of the fixing belt  1 . The pressing resistance member  4  has a shape such that both end portions  4   a  thereof project from side edges of the fixing belt  1 . Both end portions  4   a  are supported fixedly by frames  6  in the fixing device and resist the pressing force acting thereon from the pressing roller  2  through the fixing belt  1  and the heater  3 . 
         [0030]    Guide members  7  for the fixing belt are fixed at positions near both ends of the pressing resistance member  4  and corresponding to side edges of the fixing belt  1 . Sliding portions  7   a  of the guide members  7  are brought into abutment against the inner periphery surface of the fixing belt  1  in the vicinity of the side edges of the fixing belt to restrain the shape of the fixing belt  1  when moved in the circumferential direction, thereby permitting a smooth drive. 
         [0031]    It is necessary for the material of the pressing resistance member  4  to have a rigidity such that the amount of deflection upon receipt of a pressing force from the pressing roller  2  is below an allowable level, which may be below 1 mm. A heat-resisting resin such as, for example, glass fiber-filled PPS (polyphenylene sulfide), phenol, polyimide, or a liquid crystalline polymer may be used. 
         [0032]    As shown in  FIGS. 1 and 2 , the heater  3 , which is an exemplary embodiment of the present invention, is supported fixedly by the pressing resistance member  4  and is abutted against the inner periphery surface of the fixing belt  1  to heat the fixing belt  1  directly. The heater  3  includes a hollow tube  31  formed of metal such as aluminum alloy or SUS. As shown in  FIG. 4 , the tube  31  has a flat sectional shape and an electron source  32  is supported fixedly in the interior of the tube  31 . The flat section of the hollow tube  31  provides portions  31   a  and  31   b  whose peripheral surfaces are close to each other. The portions  31   a  and  31   b  form substantially flat surfaces. The electron source  32  is supported by the inner periphery surface of one flat portion  31   a  through an insulating layer  33  and the outer periphery surface of this portion is fixed to the pressing resistance member  4 . The outer periphery surface of the other flat portion  31   b  is put in abutment against the fixing belt  1  and the fixing belt  1  is heated by heat conduction from this portion. 
         [0033]    The electron source  32  is supported fixedly by one of the flat peripheral surface portions of the tube  31  through the insulating layer  33  and is opposed through a gap of about 1 mm to the inner periphery surface of the tube  31  on the side where the tube  31  is abutted against the fixing belt  1 . As shown in  FIG. 5 , the electron source  32  includes a metallic electrode  32   a  and a coating layer  32   b  formed on the electrode  32   a.  The coating layer  32   b  is formed with very small projections  32   c  of a nano-scale formed of carbon nanotubes. To be exact, the carbon nanotubes are soot containing carbon nanotubes. Components of the soot include carbon nanotubes, amorphous carbon, small pieces of graphene, and nickel or yttrium as a catalyst metal for the formation of carbon nanotubes. 
         [0034]    The coating layer  32   b  may be formed in the following manner. 
         [0035]    Soot containing nanotubes described above is pulverized by means of a mixer and mixed with liquid ethanol to prepare a suspension. Then, the suspension is sprayed to the electrode  32   a  in the electron source  32 . Thereafter, an adhesive tape is affixed to the thus-coated surface and is peeled off, whereby the very small projections  32   c  of nanotubes can be formed. 
         [0036]    There also may be adopted another method for forming the very small projections  32   c  on the coating layer  32   b.    
         [0037]    The interior of the tube  31  is pressure-reduced and bias voltage making the electron source  32  negative in polarity is applied between the electron source  32  and the tube  31  from a power supply unit  8 . For example, voltage at which the electron source  32  becomes relatively negative in polarity may be applied between the electron source  32  and the tube  31 , or the tube  31  may be electrically connected to ground and potential of a negative polarity may be applied to the electron source  32 . 
         [0038]    By the application of such bias voltage the electrons emitted from the electron source  32  move to the positive polarity side within the pressure-reduced tube and jump to the opposite flat portion (heating surface)  31   b  of the tube  31 . The energy of the electrons become heat energy on the heating surface  31   b  and is heated. 
         [0039]    As to the above pressure-reduced state, it suffices for the state to be a removed state of remaining gas to such an extent as does not obstruct the flying of electrons. More particularly, it suffices for the state in question to be a pressure-reduced state to 10 −3  Pa or lower. 
         [0040]    In this embodiment the gap between the electron source  32  and the heating surface  31   b  of the tube is set at 1 mm, but the smaller the gap, the larger the amount of electrons which are field-emitted and the more rapidly is it possible to effect heating. Therefore, the heating speed can be controlled by adjusting the gap. 
         [0041]    Moreover, the higher the negative bias voltage in the electron source  32  relative to the heating surface  31   b,  the larger the amount of electrons which are field-emitted and the higher the heating speed is. Therefore, also by adjusting the voltage to be applied between the electron source  32  and the tube  31  it is possible to control the heating speed and the heating temperature. 
         [0042]    The electron source  32  can be heated also by supplying an electric current into an electrode, whereby thermions are emitted and the heating surface  31   b  of the tube  31  can be heated rapidly. The electron source may be of the type having an electron extraction electrode, i.e., grid, and emit electrons as a cold cathode. 
         [0043]    As shown in  FIG. 4B , the interior of the tube  31  is divided into plural regions axially by means of a partition wall  31   c  disposed in the interior of the tube  31 . The electron source  32  is mounted in each of the divided regions through the insulating layer  33 . With switching elements  34 , bias voltage can be applied independently to each electron source  32 , that is, the generation of heat can be controlled independently for each divided region. According to this construction, even when heat is robbed of by a recording sheet P in the passing region of the recording sheet in accordance with the size of the recording sheet which is fed into the fixing device and there arises a temperature difference between the recording sheet passing region and the other region, i.e., non-passing region of the sheet, it is possible to keep the temperature of the paper passing region appropriate and prevent overheating of the non-passing region. 
         [0044]    Next, a description will be given below about the operation of the fixing device. 
         [0045]    In an image forming section, a toner image T is formed using four-color toners of yellow, magenta, cyan and black and in accordance with an image signal and is transferred onto the recording sheet P by means of a transfer unit (not shown). The toners are each made up of a binder of a thermoplastic resin and a colored pigment contained therein. 
         [0046]    On the other hand, almost at the same time when the toner image forming operation is started, the motor  10  for driving the pressing roller  2  is turned ON and the pressing roller  2  is rotated. With the rotation of the pressing roller  2  the fixing belt  1  moves in the circumferential direction. 
         [0047]    Further, voltage is applied between the electron source  32  and the tube  31  in the heating  3 . Consequently, the electrons field-emitted from the electron source  32  move toward the opposite peripheral surface of the tube  31  as indicated by arrows B in  FIG. 4  and the electrons-reached portion is heated rapidly. 
         [0048]    The recording sheet P which carries the unfixed toner image T is superimposed on the fixing belt  1  so that the toner image T is abutted against the belt, and is fed to the nip portion where the pressing roller  2  is pressed against the recording sheet P. In the nip portion, the fixing belt  1  and the recording paper P are brought into strong pressure contact with each other at the position between the pressing roller  2  and the heater  3 . As a result, heat is conducted from the heater  3  to the toner image T through the fixing belt  1  and the toner thereby softens and is brought into pressure contact onto the recording sheet. 
         [0049]    The heater  3  has a flat sectional shape, so in the nip portion it is brought into pressure contact with the fixing belt  1  over a wide flat surface portion thereof and the toner image T is headed and pressurized in this range. Thus, the toner image is heated sufficiently and is fixed to a satisfactory extent. In the flat peripheral surface portion of the tube  31  the electron source  32  and the inner peripheral surface of the tube  31  are opposed to each other at a substantially equal spacing and this area is heated almost uniformly, so that the toner image heating temperature is controlled appropriately and fixing is performed to a satisfactory extent without unevenness in luster or offset. Besides, heating can be done limitedly to the circumferential area of the tube  31  which area comes into abutment against the fixing belt  1  and thus efficient heating can be effected. 
         [0050]    Moreover, the internal space of the tube  31  is divided axially, and therefore, when using a recording sheet of a small size, the controller  60  in the image forming apparatus sets the time for heating only the recording sheet passing region and maintains this region at a temperature suitable for fixing, whereby it becomes possible to prevent overheating of the region where the recording sheet does not pass. 
         [0051]    On the other hand, since the electron source  32  incorporated in the heater  3  is opposed in proximity over a wide range to the inner periphery surface of the tube  31 , it is possible to emit a large amount of electrons even at a low voltage and heat the tube  31  rapidly over the wide range. 
         [0052]    Further, the very small projections  32   c  are formed on the coating layer  32   b  of the electrode  32   a  in the electron source  32  and the tip size thereof is on the order of nanoscale. Therefore, a strong field concentration occurs at the tip portion of each of the very small projections  32   c  and electrons are emitted by the application of a low voltage, whereby it is possible to achieve a high-frequency heating. 
         [0053]    The foregoing description of the exemplary embodiments of the present invention has been provided for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise forms disclosed. Obviously, many modifications and variations will be apparent to practitioners skilled in the art. The exemplary embodiments were chosen and described in order to best explain the principles of the invention and its practical applications, thereby enabling others skilled in the art to understand the invention for various embodiments and with the various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the following claims and their equivalents.