tailieunhanh - Science Technology Carbon Nanotubes Episode 8

Tham khảo tài liệu 'science technology carbon nanotubes episode 8', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 130 2 CNT Synthesis The standard method to synthesize MWCNTs is based on the electric-arc experiment proposed by Ebbesen and Ajayan 8 . Basically the production system is similar to the one used by Krãtschmer et al. 11 to produce macroscopic quantities of Cộ0 and the main difference between the two experiments is the inert gas pressure that must be rather low 20-100 mbar for an efficient fullerene production 11 but must be increased to 350-700 mbar to generate nanotubes efficiently 8 Our experimental set-up uses 6 mm graphite electrodes and the DC applied voltage is generated by an AC power supply and rectifying diodes. We have generally used a voltage of 18-20 Volts and a stable discharge is obtained for currents of 60-80 A. In our set-up the inert-gas atmosphere is static which means that the helium is introduced at the beginning 350 mbar and then the evaporation chamber is closed during the experiment the pressure increases and it usually attains 450 mbar after 10-15 min. During the discharge one electrode is moved in such a way that the discharge remains stable this can be monitored by the current value . After this period a deposit 10-15 mm long forms on the cathode which is composed of a hard grey shell formed and a black inner core. An eye observation of the black core easily reveals a columnar texture in the direction of the deposit growth. The columns are actually formed by bundles of CNTs. Some details of the arc-discharge process must be considered when analysing the quality of the generated samples. The electric arc is basically dynamic and the regions where the discharge originates moves constantly over the electrode surface. As a consequence important temperatures variations are produced and this fact is probably at the origin of the large CNT size distributions. Figure 1 shows some of the typical structural parameters of CNT generated in our laboratory. On the average we can think that an MWCNT is a 1 micron long structure formed by about 10-15 .