Saturday, November 21, 2009

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.


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