Low-temperature synthesis of carbon nanotubes via floating catalyst-chemical vapor deposition method

Carbon nanotubes (CNTs) are widely synthesized at high temperatures via floating catalyst chemical vapor deposition (FC-CVD) method. It is important to reduce the synthesis temperature of CNTs to allow better control of the reactor’s conditions, and to eliminate the formation of carbon by-products as well as reduce the overall cost. Therefore, the main objectives of this work were (i) to synthesize carbon nanotubes at low temperatures using some improvements to the CVD technique, (ii) to investigate the effects of temperature on the synthesis of CNTs and (iii) to simulate the temperature and velocity profile of the FC-CVD system in the absence of reaction. The synthesis temperature of CNTs was examined in the range between 500oC and 600oC at 10oC interval. The preheating set temperature was varied between 150oC and 400oC at 50oC interval. All experiments were conducted at 1 atm and exposed sections were insulated with glass wool covered with aluminum foil. Three O-ring heating mantels were used as a preheater and three ceramic boats were used to collect the product. Temperature in-situ monitoring device was used to monitor the temperature profile in the reactor and provide the exact time to heat up the catalyst and, thus, initiate the reaction. A single heat source for both catalyst and reactor was employed to enhance the growth of CNTs. COMSOL Multiphysics was used to simulate the velocity and temperature profiles of the system in the absence of reaction. The morphology and internal structures of the CNTs formed were analyzed via Scanning Electron Microscope (SEM), Transmission Electron Microscope (TEM) and High Resolution Transmission Electron Microscope (HRTEM). The thermal stability and the purity of the products were analyzed by Thermal Gravimetric Analyzer (TGA). The results of the work showed that CNTs were formed at the synthesis temperature ranging from 530oC to 600oC. Both quality and quantity of CNTs synthesized were increased with the increase in synthesis temperature. The highest amount of carbon deposit was obtained when the preheating and the synthesis temperatures were set at 300oC and 600oC, respectively. However, based on the TGA results, the highest purity of CNTs was achieved when the preheating and the synthesis temperatures were set at 200oC and 600oC, respectively. Well aligned CNTs were found when the preheating temperature was set at 400oC for synthesis temperatures of 570oC and 580oC. The use of single heat source for both catalyst and reactor was found to induce the formation of well aligned CNTs at synthesis temperature of 550oC. The simulation results indicated some regions in the reactor have variation in both temperature and velocity profiles. This, indirectly, affects the formation of CNTs. To conclude, this work has successfully achieved the outlined objectives. The amount of product produced and its quality depend both on the preheating values and the synthesis temperature.

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