Tham khảo tài liệu 'carbon nanotubes episode 4', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 48 . Kiang el al. tubes growing radially urchin style for cc-Ni or N1C3 particles in the rubbery collar that forms around the cathode have also been found 16 17 . Second despite the fact that copper reportedly does not catalyze single-layer tube growth in the gas phase ll Lin el al. found that numerous short single-layer tube structures form on the cathode tip when a Cu-containing anode is used 18 . Finally the growth of single-layer tubules on a graphite substrate by pyrolyzing a hydro-gen benzene mixture in a gas-phase flow-reactor at 1000 C was recently reported 19 . That experimental result is unique among those described here in that no metal atoms are involved. An overview of some of the experimental results on single-layer nanotubes is presented in Table 1. In the arc-production of nanotubes experiments to date have been carried out in generally similar fashion. An arc is typically run with a supply voltage of 20-30 V and a DC current of 50-200 A depending on the electrode diameters which range from 5-20 mm . Usually He buffer gas is used at a pressure in the range 50-600 Torr and flowing at 0-15 ml min. The anode is hollowed out and packed with a mixture of a metal and powdered graphite. In addition to pure iron group metals mixtures of these metals 7 8 10 and metal compounds oxides carbides and sulfides 5 have been successfully used as source materials for the catalytic metals in nanotube synthesis. The ratio of metal to graphite is set to achieve the desired metal concentration typically a few atomic percent. Parameter studies have shown that single-layer nanotubes can be produced by the arc method under a wide range of conditions with large variations in variables such as the buffer gas pressure 100-500 Torr gas flow rate and metal concentration in the anode 4 5 . These parameters are found to change the yield of nanotubes but not the tube characteristics such as the diameter distribution. In contrast the presence of certain additional elements .
