Abstract:
A color field emission display includes a sealed container and a color element enclosed in the sealed container. The color element includes a cathode, an anode, a phosphor layer and a carbon nanotube string. The anode is located spaced from the cathode. The phosphor layer is formed on an end surface of the anode. The carbon nanotube string has a first end electrically connected to the cathode and an opposite second end functioning as an emission portion. The second end includes a plurality of tapered carbon nanotube bundles.

Description:
RELATED APPLICATIONS 
     This application is a continuation application of U.S. patent application Ser. No. 12/069,300, filed Feb. 8, 2008, entitled, “COLOR FIELD EMISSION DISPLAY HAVING CARBON NANOTUBES”. 
    
    
     BACKGROUND 
     1. Field of the Invention 
     The invention relates to color field emission displays and, particularly, to a color field emission display having carbon nanotubes. 
     2. Discussion of Related Art 
     Field emission displays (FEDs) are based on emission of electrons in vacuum. Electrons are emitted from micron-sized tips in a strong electric field, and the electrons are accelerated and collide with a fluorescent material, and then the fluorescent material emits visible light. FEDs are thin, light weight, and provide high levels of brightness. 
     Carbon nanotubes (CNTs) produced by means of arc discharge between graphite rods were first discovered and reported in an article by Sumio Iijima, entitled “Helical Microtubules of Graphitic Carbon” (Nature, Vol. 354, Nov. 7, 1991, pp. 56-58). CNTs also feature extremely high electrical conductivity, very small diameters (much less than 100 nanometers), large aspect ratios (i.e. length/diameter ratios) (greater than 1000), and a tip-surface area near the theoretical limit (the smaller the tip-surface area, the more concentrated the electric field, and the greater the field enhancement factor). These features tend to make CNTs ideal candidates for electron emitter in FED. Generally, a color FED of the FED includes a number of CNTs acting as electron emitters. However, single CNT is so tiny in size and then the controllability of the method manufacturing is less than desired. Further, the luminous efficiency of the FED is low due to the shield effect caused by the adjacent CNTs. 
     What is needed, therefore, is a color FED, which has high luminous efficiency and can be easily manufactured. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Many aspects of the present color FED can be better understood with reference to the following drawings. The components in the drawings are not necessarily to scale, the emphasis instead being placed upon clearly illustrating the principles of the present color FED. 
         FIG. 1  is a schematic, top-sectional view of a color FED according to an embodiment. 
         FIG. 2  is a schematic, cross-sectional view of a color FED according to an embodiment. 
         FIG. 3  is a schematic, amplificatory view of part  210  in  FIG. 2 . 
         FIG. 4  is a Scanning Electron Microscope (SEM) image, showing part  210  in  FIG. 2 . 
         FIG. 5  is a Transmission Electron Microscope (TEM) image, showing part  210  in  FIG. 2 . 
     
    
    
     Corresponding reference characters indicate corresponding parts throughout the several views. The exemplifications set out herein illustrate at least one preferred embodiment of the color FED, in one form, and such exemplifications are not to be construed as limiting the scope of the invention in any manner. 
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Reference will now be made to the drawings to describe the preferred embodiments of the present color FED having carbon nanotubes, in detail. 
     Referring to  FIGS. 1 and 2 , a color FED  100  includes a sealed container  10  having a light permeable portion  12 , and at least one color element  20  enclosed in the sealed container  10 . The sealed container  10  is a hollow member that defines an inner space in vacuum. The cross section of the sealed container  10  has a shape selected from a group consisting of circular, ellipsoid, quadrangular, triangular, polygonal and so on. The sealed container  10  may be comprised of any nonmetallic material, and the emission portion  12  need be made of a transparent material. In the present embodiment, the sealed container  10  is a hollow cylinder and comprised of quartz or glass. A diameter of the sealed container  10  is about 2-10 millimeters (mm), and a height thereof is about 5-50 mm. The light permeable portion  12  has a surface selected from the group consisting of a plane surface, a spherical surface and an aspherical surface. Due to at least one color element  20  being sealed into one sealed container  10 , the method for manufacturing the color FED  100  is simple and convenient, and the luminescence efficiency thereof is improved. 
     Each color element  20  includes a cathode  24 , three anodes  28 , three phosphor layers  26  and three CNT strings  22 . The distances between the cathode  24  and the anodes  28  are substantially equal, and are about 0.1-10 millimeters (mm) The spaces among the adjacent anodes  28  are beneficially equal. The cathode  24  is electrically connected to a cathode terminal  214 , and each of the anodes  28  is electrically connected to a corresponding anode terminal  216 . The cathode terminal  214 , and the anode terminal  216  run from the inside to the outside of the sealed container  10 , and are supplied with the power source. By adjusting the voltages applied to the anode terminals  216 , the color FED  100  can emit any kinds of color light beam, such as white, yellow. The cathode  24 , the anodes  28 , the cathode terminal  214  and the anode terminals  216  are made of thermally and electrically conductive materials. 
     In each color element  20 , the anodes  28 , the phosphor layers  26  and the CNT strings  22  have the same structures, and thus the cathode  24 , the anode  28 , the phosphor layer  26  and the CNT string  22  are described in the following as an example. Referring to  FIG. 2 , the phosphor layer  26  with a thickness of about 5-50 microns (pm) is formed on a end surface  212  of the anode  28 . The phosphor layer  26  may be a white phosphor layer, or a color phosphor layer, such as red, green or blue. The end surface  212  is a polished metal surface or a plated metal surface, and thus can reflect the light beams emitted from the phosphor layer  26  to the permeable portion  12  to enhance the brightness of the color FED  100 . 
     The CNT string  22  is electrically connected to and in contact with the cathode  24  by a conductive paste, such as silver paste, with an emission portion  210  of the CNT string  22  suspending. The phosphor layer  26  is opposite to the light permeable portion  12 , and the emission portion  210  is corresponding to the phosphor layer  26 . A distance between the emission portion  210  and the phosphor layer  26  is less than 5 mm. The emission portion  210  can be arranged perpendicular to the phosphor layer  26 , parallel to phosphor layer  26  or inclined to phosphor layer  26  with a certain angle. In the present embodiment, the emission portion  210  is parallel to phosphor layer  26 , and arranged between the phosphor layer  26  and the light permeable portion  12 . The cathode  24  is made of an electrically conductive material, such as nickel, copper, tungsten, gold, molybdenum or platinum. 
     The CNT string  22  is composed of a number of closely packed CNT bundles, and each of the CNT bundles includes a number of CNTs, which are substantially parallel to each other and are joined by van der Waals attractive force. A diameter of the CNT string  22  is in an approximate range from 1 to 100 microns (μm), and a length thereof is in an approximate range from 0.1-10 centimeters (cm). 
     Referring to  FIGS. 3 ,  4  and  5 , the CNTs at the emission portion  210  form a tooth-shaped structure, i.e., some of CNT bundles being taller than and projecting above the adjacent CNT bundles. Therefore, a shield effect caused by the adjacent CNTs can be reduced. The voltage applied to the CNT string  22  for emitting electrons is reduced. The CNTs at the emission portion  210  have smaller diameter and fewer number of graphite layer, typically, less than 5 nanometer (nm) in diameter and about 2-3 in wall. However, the CNTs in the CNT string  22  other than the emission portion  210  are about 15 nm in diameter and more than 5 in wall. 
     A method for making the CNT string  22  is taught in U.S. Application No. US16663 entitled “METHOD FOR MANUFACTURING FIELD EMISSION ELECTRON SOURCE HAVING CARBON NANOTUBES”, which is incorporated herein by reference. The CNT string  22  can be drawing a bundle of CNTs from a super-aligned CNT array to be held together by van der Waals force interactions. Then, the CNT string  22  is soaked in an ethanol solvent, and thermally treated by supplying a current thereto. After the above processes, the CNT string  22  has improved electrical conducting and mechanical strength. 
     In operation, a voltage is applied between the cathode  24  and the anode  28  through the cathode terminal  214  and the anode terminal  216 , an electric field is formed therebetween, and electrons are emanated from the emission portion  210  of the CNT string  22 . The electrons transmit toward the anode  28 , hit the phosphor layer  26 , and the visible light beams are emitted from the phosphor layer  26 . One part of the light beams transmits through the light permeable portion  12 , another part is reflected by the end surface  212  and then transmits out of the light permeable portion  12 . Using the CNT string  22 , the luminance of the color FED  100  is enhanced at a relatively low voltage. 
     The color FED  100  may further includes a getter  14  configured for absorbing residual gas inside the sealed container  10  and maintaining the vacuum in the inner space of the sealed container  10 . More preferably, the getter  14  is arranged on an inner surface of the sealed container  10 . The getter  14  may be an evaporable getter introduced using high frequency heating. The getter  14  also can be a non-evaporable getter. 
     The color FED  100  may further includes an air vent (not shown). The air vent can be connected with a gas removal system such as, for example, a vacuum pump for creating a vacuum inside the sealed container. The color FED  100  is evacuated to obtain the vacuum by the gas removal system through the air vent, and then sealed. 
     Finally, it is to be understood that the above-described embodiments are intended to illustrate rather than limit the invention. Variations may be made to the embodiments without departing from the spirit of the invention as claimed. The above-described embodiments illustrate the scope of the invention but do not restrict the scope of the invention.