Abstract:
An apparatus includes a substrate, charged particles, a transparent electrode disposed in parallel with a surface of the substrate, an electrode disposed at a periphery of the transparent electrode, means for applying a voltage between the transparent electrode and the electrode disposed at the periphery thereof to move the charged particles laterally, a reflection surface for reflecting light incident onto in the substrate, and a light scattering or absorption layer disposed between the transparent electrode and the reflection surface. The apparatus is effective in providing a desired scattering ability without increasing a drive voltage.

Description:
FIELD OF THE INVENTION AND RELATED ART  
       [0001]     The present invention relates to a display apparatus, particularly an electrophoretic display apparatus which effects display based on movement of charged particles.  
         [0002]     Electrophoretic display apparatuses having various structures have been conventionally proposed with respect to such an electrophoretic display apparatus for performing display based on movement of charged particles.  
         [0003]     For example, Japanese Laid-Open Patent Application (Tokkaihei) 11-202804 and (Tokuhyo) 2002-504696 have disclosed an electrophoretic display apparatus in which an insulating liquid, charged particles and electrodes are disposed between a pair of substrates disposed with a spacing therebetween and the charged particles are moved by applying a voltage to the electrodes.  
         [0004]     The electrophoretic display apparatuses includes those of a reflection type and a transmission type. In the case of the reflection type electrophoretic display apparatus, a scattering layer is disposed on a rear substrate so as to scatter light incident from an image viewer side.  
         [0005]     The scattering layer may preferably have a larger thickness in order to improve a resultant luminance.  
         [0006]     However, the scattering layer has insulating properties and is disposed between electrodes, so that a drive voltage is increased if the thickness of the scattering layer is larger. As a result, there is a possibility of an increase in power consumption.  
       SUMMARY OF THE INVENTION  
       [0007]     An object of the present invention is to provide a display apparatus capable of suppressing an increase in drive voltage.  
         [0008]     Another object of the present invention is to provide a display apparatus capable of improving a luminance.  
         [0009]     According to the present invention, there is provided apparatus including a substrate, charged particles, a transparent electrode (first electrode) disposed in parallel with a surface of the substrate, an electrode (second electrode) disposed at a periphery of the transparent electrode, means for applying a voltage between the transparent electrode and the electrode disposed at the periphery thereof to move the charged particles laterally, a reflection surface for reflecting light externally incident onto the substrate, and a light scattering or absorption layer (first scattering layer) disposed between the transparent electrode and the reflection surface.  
         [0010]     In the apparatus, the scattering layer is disposed between the first electrode (transparent electrode) and the reflection surface. Accordingly, it is possible to narrow a spacing between the first electrode and the second electrode, so that a drive voltage becomes small. In the present invention, another (second) scattering layer may be disposed between the first and second electrodes. In this case, the resultant scattering layer can be made thin, so that it is possible to suppress an increase in drive voltage. Further, the entire thickness of the scattering layers can be made thick by the presence of the first scattering layer. As a result, a resultant luminance is improved.  
         [0011]     These and other objects, features and advantages of the present invention will become more apparent upon a 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  
       [0012]      FIGS. 1 and 2  are respectively a schematic sectional view showing an embodiment of an apparatus (electrophoretic display apparatus) according to the present invention.  
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0013]     Hereinbelow, embodiments of the apparatus according to the present invention will be described with reference to  FIGS. 1 and 2 .  
         [0014]      FIG. 1  shows an embodiment of an electrophoretic display apparatus as the apparatus of the present invention.  
         [0015]     Referring to  FIG. 1 , an electrophoretic display apparatus D 1  includes a pair of first and second substrates  1   a  and  1   b , a scattering layer  2  disposed on the second substrate  1   b , a plurality of charged particles  4  and an insulating liquid  3  which are disposed between the first substrate  1   a  and the scattering layer  2 , and a first electrode  5   a  and the second electrode  5   b  which are disposed in the vicinity of the insulating liquid  3 . In the apparatus D 1 , the charged particles  4  are moved by applying a voltage between the first and second electrodes  5   a  and  5   b  to effect display. By the movement of the charged particles  4 , two display states including a state wherein the second electrode  5   a  is covered with the charged particles  4  and a state wherein the second electrode  5   a  is exposed are generated.  
         [0016]     The above-described scattering layer  2  is used to scatter light B and includes a first scattering layer  2   a  disposed between the second electrode  5   b  and the insulating liquid  3  and a second scattering layer  2   b  disposed to sandwich the second electrode  5   b  with the first scattering layer  2   a . The thickness of the first scattering layer  2   a  is set in the range of 0.1-20 μm, and the thickness of the second scattering layer  2   b  is set in the range of 100- 1000 μm. As a material for these scattering layers  2   a  and  2   b , it is possible to use a transparent insulating resin containing high-reflective fine in particles. As the fine particles, those of titanium oxide and Al 2 O 3  can be used. As the transparent resin, it is possible to use acrylic resin, urethane resin, fluorine-containing resin, norbornene resin, polycarbonate (PC), polyethylene terephthalate (PET), etc.  
         [0017]     In the case where the scattering layer  2  ( 2   a ,  2   b ) has a sufficient thickness or strength, as indicated by reference numerals  12   a ,  12   b ,  22   a  and  22   b  in  FIG. 2 , the scattering layers  12   a  and  12  ( 22   a  and  22   b ) may be disposed to support a second electrode  15   b  ( 25   b ) or seal up the insulating liquid  3 . If the scattering layer  2  does not have a sufficient thickness or strength, as shown in  FIG. 1 , the scattering layer  2  may be supported by the second substrate  1   b  disposed in parallel with the first substrate  1   a . In this case, a reflection layer (not shown) may be disposed between the scattering layer  2  and the second substrate  1   b  so as to reflect light.  
         [0018]     The scattering layer  2  may be colored for color display. For example, scattering layers colored three colors of red (R), green (G) and blue (B) may be regularly arranged to constitute a color display apparatus. Alternatively, the surface of the second substrate  1   b  may be made a diffuse reflector (diffuse reflection surface) so as to cause light scattering at the substrate surface, so that the scattering layers in  2   a  and  2   b  may be modified to simple color filter layers (having no light scattering function). In either case, the lower scattering layer  2   b  can be made thicker, so that it is possible to arbitrarily set a light scattering function or a color filter function depending on the thickness of scattering layer.  
         [0019]     Further, between adjacent two pixels A (only one pixel A is shown in  FIGS. 1 and 2  for convenience), a partition member  6  is disposed so as to partition the pixels A, thus preventing the insulating liquid  2  or the charged particles  4  from moving into another (adjacent) pixel. The partition member  6  shown in  FIGS. 1 and 2  is a plate-like member extending in a thickness direction of the first and second substrates  1   a  and  1   b  but may have a microcapsule shape.  
         [0020]     In the case of the electrophoretic display apparatus D 1  shown in  FIG. 1 , the first electrode  5   a  is disposed at a boundary portion of the pixel A, i.e., between the partition member  6  and the scattering layer  2   a , and the second electrode  5   b  is disposed over the entire pixel A. In this case, the second electrode  5   b  may preferably be formed of a light transmissive material. As a result, light B incident from the first substrate  1   a  side passes through the insulating liquid  3  and the second electrode  5   b  and is reflected or scattered by the first scattering layer  2   a  and/or the second scattering layer  2   b . Incidentally, the second electrode  5   b  may be formed of a non-light transmissive material. In such a case, however, as shown in  FIG. 2 , the second electrode  15   b  is required to be disposed in a narrow area.  
         [0021]     In order to control a distribution sate of the charged particles  4 , it is necessary to appropriately form a potential gradient between the first electrodes  5   a , . . . and the second electrodes  5   b , . . . For example, in order to permit visual identification of only a color of colored charged particles  4 , the charged particles  4  may be distributed so as to cover the scattering layers  2 , . . . In order to realize a bright display by utilizing reflected light from the scattering layer,  2 , . . . , it is preferred that the scattering layers  2 , . . . are exposed to light.  
         [0022]     The charged particles  4  used in the present invention may preferably have an average particle size of 0.1-10 μm. A colorant (coloring agent) used in the present invention is not particularly limited. Examples thereof may include carbon black, titanium oxide, barium sulfate, nigrosine, iron black, aniline blue, calco oil blue, chrome yellow, ultramarine blue, Dupont oil red, quinoline yellow, methylene blue chloride, phthalocyanine blue, phthalocyanine green, in sky blue, rhodamine lake, etc.  
         [0023]     As a resin for the charged particles  4 , it is possible to use polymeric materials including: polyethylene-based resins such as polystyrene, polyethylene, polyester, polymethacrylate, polyacrylate, polyacrylic ester, ethylene-acrylic acid copolymer, ethylene-methacrylic acid copolymer, and ethylene-vinyl acetate copolymer; polyvinyl chloride, nitrocellulose; phenolic resin; and polyamide. These materials may be used singly or in combination of two or more species.  
         [0024]     As the insulating liquid  3 , it is possible to use an organic solvent having a low electro- conductivity, i.e., high insulating properties. Examples thereof may include: aromatic hydrocarbon solvents such as benzene, toluene, xylene and naphthenic acid-based hydrocarbons; aliphatic hydrocarbon solvents, such as hexane, cyclohexane, paraffin-based hydrocarbons, and isoparaffin-based hydrocarbons; halogenated hydrocarbons; silicone oil; and high-purity petroleum.  
         [0025]     The insulating liquid  2  may contain a charge control agent, as desired. Examples thereof may include: anionic surfactants, such as carboxylates, sulfonates, fulfates, and phosphates; cationic surfactants, such as aliphatic amine salts and their quaternary ammonium salts, aromatic quaternary in ammonium salts, and heterocyclic quaternary ammonium salts; amphoretic surfactants, such as carboxy betains, sulfobetaines, aminocarboxylates, and imidazoline derivatives; nonionic surfactants, such as those of an ester type, an ester type and a nitrogen- containing type; fluorine-containing surfactants; and reactive surfactants. These surfactants may be used singly or in mixture of two or more species.  
         [0026]     According to the above described embodiments, the scattering layer  2  is constituted by the first scattering layer  2   a  disposed between the second electrode  5   b  and the insulating liquid  3  and the second scattering layer  2   b  disposed to sandwich the second electrode  5   b  in combination with the first scattering layer  2   a . Accordingly, the first scattering layer  2   a  disposed between the first and second electrodes  5   a  and  5   b  can be made thin, so that it is possible to prevent an increase in drive voltage. Further, by the presence of the second scattering layer  2   b , the thickness of the entire scattering layer ( 2   a  and  2   b ) can be made thick. Accordingly, a resultant luminance can be improved.  
         [0027]     Hereinbelow, the present invention will be described more specifically based on Examples.  
       EXAMPLE 1  
       [0028]     An electrophoretic display apparatus D 1  having the structure shown in  FIG. 1  was prepared in in the following manner.  
         [0029]     A first substrate  1   a  and a second substrate lb were disposed with a predetermined spacing. On the surface of the second substrate  1   b , a scattering layer  2   b , a second electrode  5   b , and a scattering layer  2   a  were disposed in this order. At a boundary between adjacent pixels A, a first electrode  5   a  and a partition (wall) member  6  were disposed so as to partition the pixels A (in  FIG. 1 , only one pixel A is shown for convenience). At each pixel A, an insulating liquid  3  and charged particles  4  were disposed.  
         [0030]     As a material for the scattering layers  2   a  and  2   b , an urethane resin containing titanium oxide having a particle size of 0.5 μm was used and formed in a white scattering layer. The scattering layer  2   a  had a thickness of 3 μm and the scattering layer  2   b  had a thickness of 500 μm. The second electrode  5   b  was formed of transparent ITO (indium tin oxide) and disposed over the entire pixel. The partition member  6  was formed of epoxy-based photosensitive resin. The surfaces of the partition member  6  and the scattering layer  2  were coated with a thin transparent acrylic resin (not shown). The partition member  6  and the first substrate  1   a  were adhered to each other with a UV-curable adhesive. The insulating liquid  3  principally comprised a paraffin-based hydrocarbon, and the charged particle  4  comprised black particles of a polystyrene resin containing carbon black.  
         [0031]     When the electrophoretic display apparatus was driven, a drive voltage could be lowered to ±10 V and it was possible to realize a maximum reflectance of 45%.  
       EXAMPLE 2  
       [0032]     An electrophoretic display apparatus D 2  having the structure shown in  FIG. 2  was prepared in the same manner as in Example 1 except for the following points.  
         [0033]     The second substrate  1   b  was not used, and scattering layers  12   a  and  12   b  were used as a support instead of the second substrate  1   b . A second electrode  15   b  was formed of aluminum and disposed in an area having an areal portion of 20% per the entire pixel A are. A 1 mm-thick scattering layer  12   b  was formed of PET (polyethylene terephthalate) containing titanium oxide, and a 2 mm-thick scattering layer  12   a  was formed of an acrylic resin containing titanium oxide.  
         [0034]     When the electrophoretic display apparatus was driven, a drive voltage could be lowered to ±8 V and it was possible to realize a maximum reflectance of 46%.