Invented by Dongsheng Mao, Richard Lee Fink, Zvi Yaniv, Samsung Electronics Co Ltd
The Samsung Electronics Co Ltd invention works as followsThe present invention is directed towards cathodes, cathode materials and particle-containing carbon nanotubes. The present invention also relates to field emission devices that use a cathode from the present invention as well as manufacturing methods. The cathode is sometimes used in field emission displays. The invention also includes a way to deposit a mixed layer of CNTs, particles and particles on a substrate in order to create a cathode according to the present invention. It also contains a way to control the density of CNTs in the mixed layer to optimize its field emission properties for field emission displays.
Background for Enhanced field emission of carbon nanotubes when mixed with particles
All of the techniques mentioned above, however, are not uniform in their growth and cannot deposit large amounts of carbon nanotubes. The growth conditions also require high temperatures which makes it difficult to use them with substrates that are low temperature and inexpensive.
The density of CNTs generated by the techniques mentioned above may be too high. Researchers have discovered that high density CNTs cathodes do not exhibit the expected field emission properties because neighboring nanotubes are shielding the extracted electric fields. (Bonard, et. al., Advanced Materials vol. 13, p. 184, 2001). In order to control CNT densities, high-resolution photolithography was used by creating catalytic dot capable of growing CNTs. Phys. A, vol. 74, p. 387, 2002). The method is expensive and requires the growth of high-temperature substrates.
There is therefore a need to be in a position to harvest CNTs, and then apply them or disperse them on various substrate materials. This must be done at low temperatures. It is important to be able control the CNT density in order to optimize the field emission properties.
The present invention is directed to a new cathode used in field emission devices. It also includes methods of making this cathode and optimizing its performance by increasing the emission current and lowering the threshold emission field. This cathode is made of a material that contains carbon nanotubes and particles. Modulating the density (CNTs within the particulate matrix) of field emitters is the best way to optimize the performance of the electron emission field. The optimal CNT concentration in the mixture of CNTs and particles (cathode materials) is the one that leaves the most CNTs for emission but is not too high to interfere with each other’s performance through electrical shielding. Furthermore, such a mixture can be applied to a very wide range of materials since the processing can be done at room temperature and since the optimization of CNT concentration is substrate-independent. This method is very cost-effective, as it does not require high-resolution processing. This invention could be used in any application that uses CNT materials for field emitters.
The following detailed description will help you better understand the invention. The following will describe additional features and benefits of the invention, which are the subject of claims of invention.
The present invention is directed towards cathodes, cathode materials and particle-containing carbon nanotubes. The present invention also relates to field emission devices that use a cathode from the present invention as well as manufacturing methods. The cathode is sometimes used in field emission displays. The invention also includes a way to deposit a mixed layer of CNTs, particles and particles on a substrate in order to create a cathode according to the present invention. It also contains a way to control the density of CNTs in the mixed layer to optimize its field emission properties for field emission displays.
According to the invention, “CNTs” can be made from single-wall or multi-wall nanotubes. They also include double-wall nanotubes and carbon fibrils. These CNTs can also be purified if desired. These CNTs are available in metallic, semiconducting and semimetallic forms, as well as combinations of these. In certain embodiments, CNTs have been chemically modified or derivatized. In certain embodiments, CNTs can be metallized using the techniques described by the co-pending U.S. Patent Application Ser. No. No. “10/406/928, filed Apr.
The particles that are mixed with the carbon nanotubes can be made of any material, as long as it reduces the density of CNTs in the cathode to enhance its field emission properties when integrated into an emission device. These particles can be spherical, disk-shaped, lamellar, rod-like, or any combination thereof. These particles can have a material that is conductive, semiconducting or insulating. These materials include metals and alloys as well as polymers, dielectrics (such as clays), semiconductors, and ceramics. Materials that can be used as dielectrics include, but aren’t limited to: Al2O3, La2O3, SiO2, TiO2, WC, TiC, TiO2, TiO2, and combinations thereof. Materials that are suitable for use include GaAs, GaN and combinations thereof. Nickel, iron, chrome, alloys and combinations of these metals can be used. These particles act as a matrix for CNTs, and reduce the interaction of CNTs. This enhances the field emission properties. These particles vary in size, shape and diameter. They typically range between 1 nanometer to hundreds of millimeters.
In some embodiments, the particles can also trap or hold CNTs on a substrate or within a CNT particle matrix. Some particles, as will be explained later, can be porous. Some particles, like clays, may be layered with gaps between them. The size of these gaps depends on the state the clay is in. If the clay is saturated or fully hydrated, the gaps between the layers can be as wide as several nanometers. CNTs, or CNTs that have been functionalized, may enter the gaps or pores in the particles. It may not be necessary to do more than this. The hydrates and molecules that are between the layers of the particles can be removed by certain processes, such as drying or heating. This process can collapse layers in the particles and further hold or capture the CNTs.
In FIG. “A cathode containing CNTs and particle is shown as an example embodiment. 1. Referring to FIG. The cathode consists of a substrate 103 with which a cathode materials 106 comes into contact. The cathode is composed of CNTs and particles 104. The substrate 103 may be a glass base 100 supporting a conductive coating 102.
An embodiment in which the cathode is integrated into a field-emission display device, is shown in Figure. 2. Referring to FIG. The cathode described in the previous section can be integrated into a field emission display. The base 101 has a conductive layer 102, on which is deposited the cathode materials 106. Anode consists of a substrate 204 (which may be glass substrate 204), a conductive layer, which can be ITO and a phosphor material layer 206 to receive electrons from cathode layer 106. The layer 106 emits electrons in response to a suitable electric field between anode & cathode.
FIG. The 2nd figure shows a simplified version of an LCD display. The FIG. does not show the side walls. The side walls of FIG. 2 complete the enclosure between the cathode and anode. The spacers used to hold the gap between anode and cathode are also not shown. In normal operation the gap between anode cathode will be evacuated at pressures of around 10?6 Torr, or better vacuum. In order to form pixels on the anode, many displays use multiple independently addressable lines both on the cathode as well as the anode. FIG. FIG. Other display architectures can have three elements (anode cathode grid). In this case, the addressing columns and lines are located on the cathode; the grid is at one potential. The invention is not dependent on any particular field emission display architecture, such as single pixel, multi-pixel, triode, monochrome or color, etc .).
The density of nanotubes within the cathode material depends on the relative weights of CNTs and particles. The CNTs’ weight percentage can range from 0.1% up to 99% and, more specifically, from 40% to 60%.
In some embodiments the cathode materials (CNTs+particles), of the present invention, are in the form a layer. This layer’s area and thickness can be varied depending on the application. This layer’s thickness ranges from 10 nm up to 1 millimeter, more precisely from 100 nm up to 100?m and even further from 1?m down to 20?m.
In some embodiments, the cold cathode consists of a substrate that contains the cathode. The size and shape of the substrate is not limited, but it usually has a flat top. The substrate may be made of any material, or a combination of materials that is suitable for the substrate of the present invention. The substrate can be made of conductors or semiconductors or insulators and their combinations. In some embodiments the substrate is made up of one or more layers. In certain embodiments, the substrate is made of glass.
The cathode of the present invention, when used in field emission devices can improve the field emission process. It does this by lowering electric fields required to extract current densities of a certain value.
The method by which cathodes are manufactured in the present invention generally consists of three steps: 1) choosing an appropriate combination between carbon nanotubes, particles and other materials, 2) mixing these together, and 3) applying this mixture to a suitable substrate.
The selection of CNTs or particles will vary based on the application desired and the processing method used. “Cost considerations may also be a factor.
In some embodiments the CNTs or particles are ground before mixing. This is part of some embodiments. This grinding can be accomplished in a number of ways, including with a ball milling device such as that shown in FIG. 3. In FIG. Referring to FIG. The second wheel 304 drives a shaft 308 via a turbine assembly 305, gear assembly 306, and chain assembly 307. This milling chamber 309 is where the CNTs or particles are placed.
Mixing CNTs with particles can be achieved in many different ways. CNTs are mixed with particles in some embodiments. In some embodiments the CNTs and/or particles are pre-dispersed separately. According to the present invention pre-dispersion can include suspension or dispersion in a liquid. Solvents are any solvents that disperse the CNTs or particles according to the invention. Solvents that can be used include water, methanol (ethanol), tetrahydrofuran, CH2Cl2, cyclohexane and combinations of these. It is generally advantageous for most embodiments of this invention that the solvent can be removed easily (as with evaporation). Ultrasonication can be used in some embodiments to help disperse and/or suspend the CNTs or particles.
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