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1 CARBON-FIBER WEB STRUCTURE TYPE FIELD EMITTER ELECTRODE AND FABRICATION METHOD OF THE SAME
The present application is a division of, U.S. application Ser. No. 11/064,960, filed Feb. 25, 2005, Which claims priority from Korea Application Number 10-2004-76836, filed Sep. 24, 2004, the disclosures of Which are hereby incorporated by reference herein in their entirety.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a carbon nanotube field emitter electrode. More particularly, the present invention relates to a field emitter electrode having a novel structure Which not only improves the adhesive strength of carbon nanotubes but also exhibits improved contact resistance, and a method for fabricating the field emitter electrode.
2. Description of the Related Art
Generally, field emission devices are light sources for emitting electrons in a vacuum environment, and use the principle that electrons emitted from fine particles are accelerated by a strong electric field to collide With fluorescent substances, thus emitting light. Such field emission devices provide superior luminescence efficiency and are compact and lightWeight, compared to light sources for general illuminators, such as incandescent lamps. In addition, since field emission devices do not use heavy metals, unlike fluorescent lamps, they have the advantage of being environmentally friendly. For these reasons, field emission devices have draWn attention as next-generation light sources for various illuminators and display devices.
The performance of field emission devices is mainly determined by emitter electrodes capable of emitting a field. In recent years, a carbon nanotube (CNT) has been Widely used as an electron-emitting material for emitter electrodes having excellent electron emission properties.
HoWever, carbon nanotubes have a problem in terms of non-uniforrn groWth on a large-area substrate. In an attempt to solve this problem, carbon nanotubes groWn by a separate process are purified before adhesion to the substrate. Representative methods for fabricating a carbon nanotube emitter electrode include common printing and electrophoresis.
The fabrication of a carbon nanotube emitter electrode by printing is carried out in accordance With the folloWing procedure. First, an electrode material is coated on a smooth substrate to form an electrode layer. Then, a paste of carbon nanotubes and a silver poWder is printed on the electrode layer. The resulting structure is subjected to an annealing process to remove the resin and the solvent present in the paste. The annealed structure is subjected to taping to partially expose the carbon nanotubes to the surface.
HoWever, the conventional method has the problems that the procedure is complicated and uniform dispersion of the carbon nanotubes is diflicult, thus deteriorating the characteristics of the final field emitter electrode. In addition, suflicient physical and mechanical adherence of the paste to the underlying electrode material cannot be achieved by knoWn paste printing processes.
On the other hand, the fabrication of a carbon nanotube emitter electrode by electrophoresis is carried out by the folloWing procedure. Referring to FIG. 1, first, previously purified carbon nanotubes and a dispersant (e.g., a cationic dispersant) are mixed in an electrolytic solution. Then, a
predetermined voltage is applied to both electrodes immersed in the electrolytic solution to adhere the carbon nanotubes to a substrate formed on the cathode.
The use of electrophoresis enables the carbon nanotubes to be relatively uniformly dispersed in the electrolytic solution, and simplifies the overall procedure. HoWever, the conventional method has the problem that the carbon nanotubes are susceptible to mechanical impact because of their poor adhesive strength to the substrate.
In addition, since a large quantity of organic components remain on the electrically conductive polymer constituting the emitter electrode, they are likely to be oxidized once the emitter electrode is operated, Which largely deteriorates the field emission properties. In extreme cases, undesired gases may be generated in an electron emission space Where a vacuum is required, seriously degrading the performance of the field emission device.
Therefore, the present invention has been made in vieW of the above problems, and it is an object of the present invention to provide a novel field emitter electrode having a carbonfiber Web structure in Which carbon nanofibers contain carbon nanotubes.
It is another object of the present invention to provide a method for fabricating the field emitter electrode by electrospinning (or electrostatic spinning), the field emitter electrode having a carbon-fiber Web structure in Which carbon nanofibers contain carbon nanotubes.
In accordance With one aspect of the present invention, the above objects can be accomplished by a field emitter electrode having a carbon-fiber Web structure comprising a carbon-fiber Web layer composed of a plurality of carbon nanofibers, and a plurality of carbon nanotubes adhered to or contained in the plurality of nanofibers Wherein at least a portion of the carbon nanotube is exposed to the outside of the nanofiber.
The carbon-fiber Web layer can be formed of at least one material selected from the group consisting of polyacrylonitriles, celluloses, phenol resins, and polyimides.
Preferably, the carbon nanofibers have a larger diameter than the carbon nanotubes.
In accordance With another aspect of the present invention, there is provided a method for fabricating the field emitter electrode having a carbon-fiber Web structure by electrospinning, the method comprising the steps of: mixing a carbonizable polymer, carbon nanotubes and a solvent to prepare a carbon nanotube-containing polymer solution; electrospinning the polymer solution to form a nanofiber Web layer on a substrate; stabilizing the nanofiber Web layer such that the polymer present in the nanofiber Web layer is crosslinked; and carbonizing the nanofiber Web layer such that the crosslinked polymer is converted to a carbon fiber.
The carbonizable polymer may be at least one selected from the group consisting of polyacrylonitriles, celluloses, phenol resins, and polyimides.
The nanofiber Web layer can be composed of nanofibers having a diameter larger than the carbon nanotubes. The substrate may be an electrically conductive substrate, such as an aluminum or copper sheet.
The stabilization of the nanofiber Web layer can be carried out by oxidizing the polymer present in the nanofiber Web layer in an oxidizing atmosphere at 150~3500 C. The carbon
ization of the nanofiber Web layer can be carried out by carbonizing the crosslinked polymer in an inert atmosphere at 600~l,300° C.
BRIEF DESCRIPTION OF THE DRAWINGS
The above and other objects, features and other advantages of the present invention Will be more clearly understood from the folloWing detailed description taken in conjunction With the accompanying draWings, in Which:
FIG. 1 is a schematic vieWs of an apparatus used in a conventional method for fabricating a field emitter electrode by electrophoresis;
FIG. 2 is a floW chart illustrating a method for fabricating a field emitter electrode according to the present invention;
FIG. 3 is a schematic vieW of an electrospinning apparatus usable in a method for fabricating a field emitter electrode according to the present invention;
FIGS. 4a to 4e are scanning electron micrographs (SEM) (5,000><) of electrospun nanofiber Web structures formed in Example 1 of the present invention;
FIG. 5 is a SEM (80,000><) of a nanofiber Web structure formed in Example 1 of the present invention;
FIGS. 6a to 6c are photographs of a carbon-fiber Web structure formed in Example 1 of the present invention taken at different magnifications; and
FIG. 7 is a photograph shoWing the luminescent state of a field emission device to Which a field emitter electrode having a carbon-fiber Web structure formed in Example 1 of the present invention is applied.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The present invention Will noW be described in more detail With reference to the accompanying draWings.
FIG. 2 is a floW chart illustrating a method for fabricating a field emitter electrode according to the present invention.
Referring to FIG. 2, the method for fabricating a field emitter electrode according to the present invention is initiated by mixing a carbonizable polymer, carbon nanotubes and a solvent to prepare a carbon nanotube-containing polymer solution (S13).
Any polymer can be used in the present invention so long as it is carbonizable. The polymer may be at least one material selected from the group consisting of polyacrylonitriles, celluloses, phenol resins, and polyimides. The kind of the solvent used in the present invention can be properly selected according to the type of the selected polymer. For example, the solvent may be dimethylforrnamide (DMF), toluene, benzene, acetone, or alcohol.
The carbon nanotubes used in the present invention can be obtained by pulverizing multi-Wall or single-Wall carbon nanotubes by chemical vapor deposition (CVD) or arc discharge, and purifying the pulverized multi-Wall or single-Wall carbon nanotubes by knoWn processes, such as field-fluxfloW fractionation. The carbon nanotubes thus obtained preferably have a length of 1 pm to 2 pm, and a diameter of a feW nanometers to tens of nanometers.
Thereafter, the polymer solution is electrospun to form a nanofiber Web layer on a substrate (S15).
Generally, electrospinning is a process that has been Widely used in various industrial fields, including fibers, fuel cells, and cell electrodes, and refers to a process Wherein nanofibers having a diameter of a feW nanometers to hundreds of nanometers are spun from a polymeric precursor material by applying an electrostatic voltage to the precursor to form a
random Web structure. Similarly, When an electrostatic voltage is applied betWeen the carbon nanotube-containing polymer solution and the substrate in step (S15), nanofibers containing the carbon nanotubes are spun on the substrate and thus a desired nanofiber Web layer can be formed on the substrate.
The substrate usable in step (S15) may be a non-conductive film having a thickness small enough to alloW the applied electrostatic voltage for electrospinning, but is preferably an electrically conductive substrate, such as an aluminum or copper sheet, usable as a substrate for an emitter electrode. Details of the electrospinning process employed in the present invention are shoWn in FIG. 3.
Next, the nanofiber Web layer formed on the substrate by electrospinning is stabilized (S17). This stabilization is achieved by annealing the nanofiber Web layer in an oxidizing gas atmosphere at a predetermined temperature, and thus refers to “an oxidation process”. The annealing conditions vary depending on the kind of the polymer used, but the annealing is preferably carried out at a temperature ranging from about 150° C. to about 350° C. for 2~5 hours. At this step, the polymer present in the nanofiber Web layer is crosslinked by the action of oxygen, making the nanofiber Web layer stable.
Finally, the stabilized nanofiber Web layer is carbonized (S19). In step (S19), this carbonization is achieved by annealing the stabilized nanofiber Web layer in an inert atmosphere at a predetermined temperature, and refers to a step Wherein organic components remaining after formation of a hexagonal graphite structure are removed from the crosslinked polymer, and the crosslinked polymer is converted to a carbon fiber. The carbonization is preferably carried out in an inert gas atmosphere, e.g., nitrogen, at 600~l ,300° C. for about 0.5 hours to about 1 hour. Since organic components remaining after formation of a hexagonal graphite structure are removed from the crosslinked polymer in step (S19), the problem encountered With remaining organic components in the prior art using a conductive polymer can be solved. In addition, additional annealing may be carried out to activate the carbon nanofibers.
As demonstrated above, according to the method of the present invention, an emitter electrode having a carbon-fiber Web structure can be fabricated by electrospinning. Although the electrospinning has been mainly employed to prepare a carbon fiber for use in fuel cells and cell electrodes, an emitter electrode having a carbon-fiber Web structure can be fabricated by electrospinning a carbon nanotube-containing polymer solution.
In the emitter electrode having a carbon-fiber Web structure according to the present invention, since the carbon nanotubes are fixed in the carbon nanofibers or the carbon-fiber Web structure, they have strong adhesive strength. In addition, since the emitter electrode of the present invention has a basic structure consisting of the highly conductive carbon nanofibers With a large specific surface area, the contact resistance can be greatly increased.
FIG. 3 is a schematic vieW of an electrospinning apparatus usable in the method of the present invention. It Will be understood that the apparatus is provided to carry out the step (S15) shoWn in FIG. 2.
As shoWn in FIG. 3, the electrospinning apparatus comprises a pipette 21, a rotating drum 22, and a high-voltage generator 23 electrically connected to both the pipette 21 and the rotating drum 22. The carbon nanotube-containing polymer solution 24 (i.e. a mixture of the carbon nanotubes, the carbonizable polymer and the solvent) is stored in the pipette 21. The polymer solution 24 is maintained at a constant level