Producing Carbon Nanotube Films In LPCVD System Using Floating Catalyst Method

Since their discovery in 1991, CNTs has become the most popular material that could have potential applications in many areas such as field emission display, fuel cell, and composites. To utilize them in fundamental investigation and industrial application, synthesizing high quality CNTs in a large scale is necessary. Thereafter, various techniques like arc-discharge, laser ablation, and chemical vapor deposition (CVD) have been developed and further improved, by which not only single-walled CNTs (SWNTs) and multi-walled CNTs (MWNTs), but also CNTs films and powder could be produced. Considering the production cost and application requirement, CVD technique is no doubt the most comprehensive one.A new approach to synthesize carbon nanotubes film has been achieved by using floating catalyst method below atmospheric pressure. Using ferrocene as a catalyst precursor, cyclohexane as carbon source, H₂ as carrier gas and thiophene as a promoter, under certain ferrocene sublimed temperature, large amounts of CNTs were synthesized in a large scale in the LPCVD system. The effect of certain experiment parameters such as pressure, H₂ flow rate ratio, ferrocene sublimed temperature and the molar ratio of C₄H₄S to C₆H₁₂ on the growth of carbon products were studied. The optimization conditions for synthesis of high quality CNTs were found after the study. Scanning electron microscopy (SEM), Raman spectroscopy and high-resolution transmission electron microscopy (HRTEM) were utilized to characterize the CNTs samples.All the CNTs are hollowed, with diameter in the range of 50 ～ 80nm.The side wall of CNTs is made of slightly tilted graphitic-like layers, with the layer-layer distance around 0.345nm, which means that the material is MWNTs. These CNTs powder could be used as field emission electron source material or CNTs-polymer composites.Carbon nanotubes are promising as new optoelectronic materials such as new electron field emitters in panel displays. For most of applications, it is highly desirable to prepare aligned carbon nanotubes (ACNTs) so that the properties of individual nanotubes can be easily assessed and they can be incorporated effectively into devices for the applications in panel field-emitting displays. We prepared aligned carbon nanotubes by the pyrolysis of iron(II) phthalocyanine, FeC₃₂N₈H₁₆ (Aldrich, designated as FePc hereafter), which contains both the metal catalyst and carbon source required for the nanotube growth. The synthesis of ACNTs was performed under Ar/H₂ flow over the temperature range 750-1000°C on quartz glass substrate. By adjusting the pressure of CVD system at low pressure with vacuum valves, FePc was effectively economized and the quality of ACNTs was developed, which was important for the potential applications. The morphology and microstructure of ACNTs were analyzed by scanning electron microscopy (SEM), field emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM). We investigated the influences of parameters such as gas pressure, growth temperature, flow rate and proportion.The field emission properties of ACNTs were measured. The result shows a very powerful ability of electron emission with a turn-on voltage of 0.67 V·μm^-1(I=1μA) and a threshold voltage of 2.5 V·μm^-1,it indicates that ACNTs are highly efficient electron field emitters.