Tsinghua University Scientists Reveal Simple Method for Continuous High Purity Carbon Nanotube Production

Tsinghua University (Beijing, CN) inventors Fei Wei, Yao Wang, Guohua Luo, Hao Yu, Zhifei Li, Weizhong Qian, Zhanwen Wang and Yong Jin reveal a method for continuous production of carbon nanotubes in a nano-agglomerate fluidized bed reactor in U.S. Patent Application 20090286675.  The carbon nanotubes produced have a purity of greater than 96% and a yield of greater than >26 g/per gram of catalyst.  By properly adjusting the reaction rate, operating conditions and fluidized-bed structure, the reactor bed is kept in an agglomerate fluidization state, so as to realize the continuous mass production of carbon nanotubes with a high degree of crystallization, high purity, and high yield.
                                      

Figure 1: Tsinghua University CNT Production Apparatus

The Tsinghua method comprises the following steps: loading transition metal compounds on a support, obtaining supported nanosized metal catalysts by reducing or dissociating, catalytically decomposing a carbon-source gas, and growing carbon nanotubes on the catalyst support by chemical vapor deposition of carbon atoms. The carbon nanotubes are 4 to 100 nm in diameter and 0.5 to 1000 microns in length. The carbon nanotube agglomerates, ranged between 1 to about 1000 microns (mu.m), are smoothly fluidized under 0.005 to 2 m/s superficial gas velocity and 20-800 kg/cubic meter bed density in the fluidized-bed reactor. The apparatus is simple and easy to operate, has a high reaction rate. During the production of the nanosized carbon materials, the distribution of temperature and concentrations in the fluidized bed are uniform, and there is neither local overheating nor coagulation. 

Normal fluidization state or even particulate fluidization state can be realized and maintained during the whole reaction process through proper control of the structure and growth of carbon nanotubes based on the analysis of the growth, agglomeration and fluidization of carbon nanotubes during the chemical vapor deposition process. The catalyst support can be selected from powders with good flowability, such as superfine glass beads, silicon dioxide, alumina and carbon nanotubes. By adopting the process, conditions and reactors of the present invention, carbon nanotubes having a loose agglomerated structure can be produced with agglomerate diameters of 1 to 1000 .mu.m, bulk density of 20 to 800 kg/per cubic meter, and with good flowability/fluidization properties.

Surprisingly, the carbon nanotubes produced using the fluidized bed with the carbon nanotube agglomerates are highly crystalline, have a purity of greater than 96% and a yield of greater than >26 g/per gram of catalyst. Moreover, in the presence of carbon nanotube agglomerates, the reaction is under a dense phase fluidization and there is no deposit of amorphous carbons. Carbon nanotubes of various structures and morphologies can be prepared using the methods and carbon nanotube agglomerates of the present invention. For example, high purity (>96%) carbon nanotubes with single-wall, double-wall, multi-wall or a mixture thereof can be prepared.

It has been more than a decade since the first report on carbon nanotube as a new material. The exceptional mechanical and electrical properties of carbon nanotube have attracted intensive attention of physicists, chemists and material scientists worldwide, however, its commercial application has not been realized yet. The reasons lie in two interrelated aspects: the difficulty in mass production of carbon nanotubes and hence the high production cost. For instance, the international market price of carbon nanotubes of 90% purity is as high as $60/g, which is 5 times that of gold. It is reported that the highest production rate of carbon nanotubes till now is only 200 g/h (MOTOO YUMURA et al., CNT10, October 2001, p. 31). There are also reports forecasting that industrial application of carbon nanotubes will remain unpractical until its price falls below $2/pound, i.e. 0.4 cent/g, and it needs a production rate of 10,000,000 pound per year or about 12.5 tons per day to bring the price down to this level. Thus, in order to take carbon nanotubes from laboratory to market, mass production of high-quality carbon nanotubes is one of the principal challenges to take. 

FIG. 3 is a Transmission Electron Microscope (TEM) photograph from the patent application of the carbon nanotubes produced using the Tsinghua University method and reaction apparatus.

FIG. 5 (from Patent Application 20090286675) shows the growth mechanism of carbon nanotube agglomerates with catalysts. Transition metal nanoparticles are formed from a transition metal oxide selected from the group consisting of Fe--Cu oxide, Ni--Cu oxide, Co--Mn oxide or Ni oxide; a solid support is selected from superfine glass beads, SiO2, Al2O3 or carbon nanotubes, wherein the metal nanoparticles and the support are combined to form  catalyst nano agglomerates; and carbon nanotubes are deposited on the catalyst nano-agglomerates in a fluidized-bed reactor.

 

Carbon Nanotube FET Device Enables Two Sided Displays

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.

Tsinghua University Carbon Nanotube Magnesium Composite Suitable for Aerospace and Auto Use

Nowadays, various alloys have been developed for special applications. Among these alloys, magnesium alloys have relatively superior mechanical properties, such as low density, good wear resistance, and high elastic modulus. However, the toughness and the strength of the magnesium alloys are not able to meet the increasing needs of the automotive and aerospace industry for tougher and stronger alloys. Tsinghua University scientists created a magnesium-based carbon nanotube composite material that is strong, tough, has a high density, and can be widely used in a variety of fields including the automotive and aerospace industries.

In the magnesium-based composite material, nanoscale reinforcements (e.g. carbon nanotubes and carbon nanofibers) are mixed with the magnesium metal or alloy. The most common methods for making the magnesium-based composite material have been thixomolding and die-casting. However, in die-casting, the magnesium or magnesium alloy is easily oxidized. In thixomolding, the nanoscale reinforcements are added to melted metal or alloy but are prone to aggregate. As such, the nanoscale reinforcements can't be well dispersed.

Tsinghua researchers Kam-Shau Chan, Cheng-Shi Chen, Guang-Liang Sheu, Qing-Chun Du And Wen-Zhen Li developed an apparatus for fabricating a magnesium-based carbon nanotube composite material, in which the problems of aggregation and settling are eliminated or at least alleviated. Tsinghua University’s method includes the steps of: (a) providing a magnesium-based melt and carbon nanotubes, mixing the carbon nanotubes with the magnesium-based melt to achieve a mixture; (b) injecting the mixture into at least one mold to achieve a preform; and (c) extruding the preform to achieve the magnesium-based carbon nanotube composite material.

The mixing device includes a container with a protective gas therein, a stirrer disposed in a center of the container, and a heater (e.g. hot wires) disposed on a outer wall of the container. Quite suitably, the protective gas can, beneficially, be made up of at least one of nitrogen, ammonia, and a noble gas. The heater heats the container to a predetermined temperature in the approximate range from 550 degree C. to 750 degree C. Carbon nanotubes are in the approximate range from 1% to 5% by weight in the alloy. The materials is extruded into performs where it cools quickly enough to prevent settling and aggregation of the carbon nanotubes.

In the extrusion step, the preforms experience a deformation process when extruded from the exit. In the deformation process, different parts of the preforms will be mixed together. The carbon nanotubes are redistributed in the preforms which enhances dispersion uniformity of the carbon nanotubes in the magnesium-based carbon nanotube composite material. The mixing apparatus is detailed in U.S. Patent Application 20090127743. It is one of 47 applications concerning carbon nanotubes filed by Tsinghua University which is among China's leaders in nanotechnology research and development. The University already hold 20 U.S. Patents related to nanotechnology.