Chinese researchers have developed a more stable and more powerful field emission device for two sided liquid crystal displays using carbon nanotubes, earning U.S Patent 7,615,919 for Tsinghua University (Beijing, CN) and Hon Hai Precision Industry Co., Ltd. (Tu-Cheng, Taipei Hsien, TW)
Field emission devices are based on emission of electrons in a vacuum. Electrons are emitted from micron-sized tips in a strong electric field, and the electrons are accelerated and collide with a fluorescent material. The fluorescent material then emits visible light. Field emission devices are thin, light weight, and provide high levels of brightness.
Conventionally, a material of the tips is selected from the group consisting of molybdenum (Mo) and silicon (Si). With the development of nano-technology, carbon nanotube (CNT) is also used in the tips of the field emission devices. However, the typical working voltage of such field emission devices is about 10,000 volts, which can easily generate enough static force to break the CNTs. As a result, performance of these field emission devices is unstable.
A more stable field emission device was developed by Yuan-Chao Yang, Jie Tang, Liang Liu, and Shou-Shan Fan. Their FET device includes a sealed container with a first light-permeable portion and an opposite second light-permeable portion. A first phosphor layer is formed on the first light-permeable portion. A first light-permeable anode is formed on the first phosphor layer. A second phosphor layer is formed on the second light-permeable portion. A second light-permeable anode is formed on the second phosphor layer. A shielding barrel is disposed within the container and electrically connected to at least one cathode electrode. One opening of the shielding barrel faces towards the first light-permeable portion and the other opening faces towards the second light-permeable portion. The shielding barrel has an inner surface. A conductive nano material layer is formed on the inner surface of the shielding barrel.
The first and the second light-permeable anodes are metal films with good electric conductivity. The anodes are aluminum films. The shielding barrel is a cylinder with a central axis perpendicular to the first and the second light-permeable portions. It can be understood that other shapes of the shielding barrel can be selected according to the shape of the sealed container.
The conductive nano-material layer comprises a material selected from the group consisting of carbon nanotubes, carbon nano-sticks, carbon nano-yarns, Buckminster-fullerenes (C60), and carbon nano-particles. The conductive nano-material layer can also be made of a material selected from the group consisting of nanotubes, nano-sticks, nano-yarns, nano-particles of conductive metal and semiconductors. The conductive nano-material layer consists of carbon nanotubes. Firstly, the nano slurry is spread on the inner surface of the shielding barrel and solidified. Then the conductive nano-material is scrubbed with a rubber to expose ends of the carbon nano tubes so that the conductivity of the shielding barrel can be enhanced. Distance between edge (e.g., top end) of the conductive nano-material layer and edge (e.g., top end) of the shielding barrel determines shielding effect of the shielding barrel.
In order to maintain the vacuum of the inner space of the sealed container, a getter may be arranged therein to absorb residual gas inside the sealed container. The getter should preferably be arranged on an inner surface of the sealed container around the electrodes. The getter may be evaporable getter introduced using high frequency heating. The getter also can be non-evaporable getter. It must be ensured that the getter does not attach to the light-permeable anodes in order to avoid short circuits between the light-permeable anode and the electrodes.
The sealed container further includes an air vent. The air vent connects a vacuum pump to vacuum the sealed container before packaging sealing the container.
In operation, when putting a voltage over the electrodes and the light-permeable anodes Electrons will emanate from two openings of the shielding barrel. The electrons move towards and transmit through the first and the second light-permeable anodes. When the electrons hit the first and second phosphor layers visible lights will be emitted. One part of the lights will transmit through the first and the second light-permeable portions, and the other part of the lights will be reflected by the first and the second light-permeable anodes, and spread out of the light-permeable portions. Tubes can be arranged together to use for lighting and two-sided displaying. Because of the shielding effect of the shielding barrel, the field emission device can operate with greater stability at higher voltages.