Patent Application
20060213774
The present invention relates generally to a method for enhancing the homogeneity and efficiency of carbon nanotube electron emission source that is manufactured by electrophoresis deposition. More particularly, the present invention relates to a method, which employs the electrophoresis deposition technology for depositing the carbon nanotube powder onto a cathode electrode, thereby forming an electron emission source. In addition, a baking process is performed after the electrophoresis deposition process to form the conductive metallic oxide salt, thus enhancing the efficiency of electron generation from the carbon nanotube electron emission layer.
Conventional triode field emission display includes an anode structure and a cathode structure. There is a spacer disposed between the anode structure and the cathode structure, thereby providing a space and a support for the vacuum region between the anode structure and the cathode structure. The anode structure includes an anode substrate, an anode conducting layer, and a phosphorus layer; while the cathode structure includes a cathode substrate, a cathode conducting layer, an electron emission layer, a dielectric layer and a gate layer. The gate layer is provided a voltage difference to induce the emission of electrons from the electron emission layer. The conducting layer of the cathode structure provides a high voltage to accelerate the electron beam, such that the electron beam can have enough kinetic energy to impinge and excite the phosphorous layer on the anode structure, thereby emitting light. Accordingly, in order to maintain the movement of electrons in the field emission display, a vacuum apparatus is required to keep the vacuum degree of the display being below 10-5 torr. Therefore, the electrons can have appropriate mean free paths. Meanwhile, the pollution and toxication of the electron emission source and the phosphorous layer should be prevented from happening. Furthermore, in order for the electrons to accumulate enough energy to impinge the phosphorous powder, a space is required between the two substrates. Consequently, the electrons can be accelerated to impinge the phosphorous layer, thereby exciting the phosphorous layer and emitting light therefrom.
The electron emission layer is composed of carbon nanotubes. Since carbon nanotubes, discovered by Iijima in 1991 (Nature, 354, 56 (1991)), comprises very good electronic properties that can be used to build a variety of devices. The carbon nanotubes also has a very large aspect ratio, mostly larger than 500, and a very high rigidity of Young moduli larger than 1000 GPn. In addition, the tips or defects of the carbon nanotubes are of atomic scale. The properties of carbon nanotubes described above are considered ideal for building electron field emitter, such as an electron emission source of a cathode structure of a field emission display. Since the carbon nanotubes comprise the physical properties described above, a variety of manufacturing process can be developed, e.g. screen printing, or thin film processing.
However, the art of manufacturing the cathode structure employs carbon nanotubes as an electron emission material, which is fabricated on the cathode conducting layer. The manufacturing process can be chemical vapor deposition process, or any kind of process that can pattern the photosensitive carbon nanotube solution on any pixel of the cathode conducting layer. Moreover, the cathode structure can also be manufactured by coating the carbon nanotubes solution incorporating with a mask. However, there are still limitations of the manufacturing cost and the cubic structure for fabricating the carbon nanotubes on each pixel of the cathode electrode structure, according to the electron emission source structure of the triode field emission display described above. In particular, the homogeneity of large size electron emission source is even harder to achieve.
Recently, a so-called electrophoresis deposition (EPD) technology has be developed and disclosed in the United States Patent Publication No. US2003/0102222A1, entitled “Deposition Method for Nanostructure Materials”. In this patent publication, the carbon nanotubes are formulated into alcoholic suspension solution. On the other hand, ionic salts of magnesium, lanthanum, yttrium, aluminum act as a charger to prepare electrophoresis solution. The cathode structure to be deposited is connected with one electrode of the electrophoresis solution. By providing a DC or AC voltage, an electric field is formed in the solution. The chargers in the solution are then dissolved into ions, so as to adhere onto the powder of carbon nanotubes. For this reason, the electric field forms an electrophoresis force to assist the carbon nanotubes depositing onto a certain electrode. In this manner, the carbon nanotubes are deposited on a patterned electrode. By using the so-called electrophoresis technology described above, the carbon nanotubes are easily deposited onto an electrode and can easily circumvent the limitations of the cathode structure of the triode field emission display. Therefore, this method is widely used in the application of cathode plate fabrication.
Although the electrophoresis deposition method has been widely adopted, part of the mechanism therein requires improvements. For example, the ions of chargers are deposited together on the electrode. In general, the anode ions react with the cathode ions (the OH cathode ions from the oxidated water molecule in the solution) on the surface of electrode to form salt hydroxide, and are deposited together with the carbon nanotubes. Since the electrophoresis process requires a repeated baking process to remove the unnecessary solvent and organic material, such metallic salt hydroxide will be converted into metallic salt oxide. Taking the conventional magnesium chloride as an example, the salts of magnesium oxide will be deposited on the electrode together with the carbon nanotubes, or on the surface of carbon nanotubes. Since the magnesium oxide is not a good conductor, although the weighted concentration thereof in the electrophoresis process is generally controlled to be below 0.1%, which does not affect the efficiency for generating electron beams from the carbon nanotube electron emission layer, it does not enhance the efficiency, either.
Another conventional art is disclosed in the Taiwanese published U.S. Pat. No. 353,188, entitled “Method for fabricating low energy electron excitation screen”, wherein a metallic salt is used to form conductive metallic oxide on the phosphorus layer after the electrophoresis process, so as to enhance the light emission property of the phosphorus powder. Accordingly, other metallic salts that are solvable in water or alcoholic solution are also employed to form conductive metallic salt oxide in the baking process after the electrophoresis deposition process, thereby enhancing the efficiency of electron production from the carbon nanotube electron emission source.
The present invention is to provide a method to overcome the drawbacks in the conventional arts described above. The present invention employs electrophoresis deposition technology to manufacture the electron emission source of the field emission display. In the present invention, the carbon nanotube powder made from arch discharge is used to prepare the electrophoresis solution. In addition, some proper chargers are used, rendering the electrolyte ions to produce good ionic distribution effect. This can enhance the distribution properties of carbon nanotubes in the water or alcoholic electrophoresis solution. The homogeneity of electrophoresis deposition on the surface of cathode electrode is thus enhanced. In addition, the properly selected charger forms conductive metallic salt oxide during the baking process after the electrophoresis deposition process, which will enhance the electron production efficiency of the carbon nanotube electron emission source layer.
In order to achieve the above and other objectives, the method of the present invention for enhancing the homogeneity and efficiency of carbon nanotube electron emission source includes the following steps. First, a semi-manufactured cathode structure is prepared. Then, the cathode structure and the metallic plate are connected to the electrophoresis electrodes.
After that, the side of the cathode structure to be electrophoresis deposited is kept a fixed distance in parallel with the metallic plate. Then, the electrophoresis deposition is performed to the semi-manufactured cathode structure by placing the combination into the solution of the electrophoresis tank. Later, an electric field is formed from a direct current voltage of a power supply. In this manner, the carbon nanotubes are deposited on the cathode electrode to form the electron emission source.
After the deposition process of the cathode structure is completed, the combination is baked with a low temperature so as to remove the residual alcoholic solution on the cathode structure. Meanwhile, the indium chloride charger and the electrolyte hydroxide ions react to form indium hydroxide. Next, a sintering process is performed for re-oxidating the indium hydroxide on the cathode electrode layer back to indium oxide. Consequently, the electron conductivity of the carbon nanotubes and the cathode electron layer is enhanced.
In order to better understanding the features and technical contents of the present invention, the present invention is hereinafter described in detail by incorporating with the accompanying drawings. However, the accompanying drawings are only for the convenience of illustration and description, no limitation is intended thereto.
Referring to
In the beginning, a cathode electrode layer 2 is formed on the surface of a glass substrate 1. A dielectric layer 3 is formed on the surface of the cathode electrode layer 2. Later, a gate electrode layer 4 is formed on the surface of the dielectric layer 3, and then forming a sagged region 41 thereon by lithography technology to expose the dielectric layer 3. Next, a protection layer 5 is formed on the surface of the gate electrode layer 41. A sagged region 31 is formed on the surface of the dielectric layer 3 exposing the cathode electrode layer 2. The protection layer 5 is then peeled off, and another protection layer 6 is coated covering the dielectric layer 3 and the gate electrode layer. Thus, a semi-manufactured cathode structure is completed.
Referring to
First, the electrophoresis solution is prepared by taking alcohol as solvent, with an additional 1%˜10% of pure water (preferably 5%), and adding the carbon nanotube powder therein. The carbon nanotube powder employed in the present invention is made from arch discharge process, with an average length of below 5 μm and an average diameter below 100 nm. Note that the carbon nanotubes are of a multi-wall structure, the weight concentration of which is approximately 0.1%˜0.005% (preferably 0.02%). In addition, a charger of weight concentration of approximately 0.1%˜0.005% (preferably 0.01%) is added to the solution. The charger can be any metallic salt oxide that induces conductivity of the electrophoresis, such as indium chloride, indium nitrate, or any other salt such as tin. The prepared solution is then poured into the electrophoresis tank 7.
After the electrophoresis solution is prepared, one can perform the electrophoresis deposition process. The cathode layer 2 of the field emission cathode structure 10 is connected to the cathode electrode 81 of the electrophoresis electrode 8 via the cathode conducting wire 101. The anode electrode 82 of the electrophoresis electrode 8 is connected to the metallic plate 9. The metallic plate 9 described above can be of platinum or titanium plate, or a screen plate.
After the cathode structure 10 and the metallic plate 9 are combined, one side of the cathode structure 10 to be electrophoresis deposited is kept parallel to the metallic plate 9 with a fixed distance, and then disposed into the electrophoresis tank 7. A direct current (DC) voltage from a power supply is provided to the cathode electrode to form an electric field. The intensity of the electric field can be 0.5˜10 V/cm and preferably 2 V/cm. The carbon nanotubes are then electrophoresis deposited on the cathode electrode 2 to form the electron emission source 21, as shown in
After completing the deposition process described above, the cathode structure is moved to an oven and baked under a low temperature of 80° C. to remove the residual alcoholic solution on the cathode structure 10. Now, the indium chloride charger and the electrolyte hydroxide form indium hydroxide. A sintering process at a temperature of 400° C. is performed to bum off the protection layer 6. The indium hydroxide on the cathode electrode layer 2 is further oxidated to form indium oxide. Since indium oxide is conductive and is remained on the surface of the electron emission source 21 in addition to the deposited carbon nanotubes after the cathode electrode 2 is manufactured, it replaces the conventional magnesium salt charger, which provides only the electrophoresis force, and enhances the electronic conduction of the carbon nanotubes and the cathode electrode layer.
Further, since the present invention employs the electrophoresis deposition method to form carbon nanotube electron emission source layer, the carbon nanotubes are easily adhered parallel to the surface of the cathode electrode forming a homogeneous layer of carbon nanotubes. The average thickness can be controlled under 2 μm, while the carbon nanotubes and the chargers of indium oxide are deposited together after sintering to form good adhesion effect without dissemination.
Even further, the carbon nanotube electrophoresis solution comprises better distribution properties than that of the conventional magnesium chloride charger. By studying the distribution properties of the conventional charger, one can find that more than 15% of an average unit area of 250 μm2 contains 10 μm carbon nanotube clusters. After changing the charger into indium chloride, the 10 μm carbon nanotube clusters can be controlled as fewer than 5%.
Since, any person having ordinary skill in the art may readily find various equivalent alterations or modifications in light of the features as disclosed above, it is appreciated that the scope of the present invention is defined in the following claims. Therefore, all such equivalent alterations or modifications without departing from the subject matter as set forth in the following claims is considered within the spirit and scope of the present invention.
