Preparation Of Carbon Nanotubes, Composite Materials, And The Field Enhancement Factor

Carbon nanotubes are just rolled graphite sheets and capped with half a fullerene at each end with diameter of the range of nanometer and with the length up to a few micrometers. Inheriting the unique properties of the graphite, such as the wearablity, self-lubrication and conductivity, carbon nanotubes also have some outstanding chemical, physical and mechanical properties. Carbon nanotubes have and could been applied in many fields, such as field emission electron sources, scanning probes, chemical sensors, field-effect transistors, nano-electronic devices, and so on, which have been the focus of the research in the condensed matter physics and materials fields in the last ten years. In this paper, a great deal of effort has gone into the synthesis and characterization of the carbon nanotubes, which included the chemical vapor deposition and are-discharge method. Then the carbon nanotubes composite materials were prepared and the mechanical properties of the composite were tested. Finally, the field emission from carbon nanotubes was calculated with the floating sphere model and numerically simulated by solving Laplace equations with computer, and the conditions of the field emission were optimized according to the results from the theoretical calculation and numerical simulation.The first chapter is the literature review. In this section, the discovery and the character of carbon nanotubes were introduced, which the unique structure leads to the outstanding properties in physical, chemical and mechanical properties. Then the development of the synthesis of the carbon nanotubes was reviewed and the technology and conditions for the growth of carbon nanotubes were discussed for the different methods, which included the chemical vapor deposition, arc discharge method and laser ablation method. Due to the different unique performance, carbon nanotubes have very promising application prospect in many field, such as field emission electron sources, scanning probes, chemical sensors, field-effect transistors, nano-electronic devices, nano-composite materials, energy storage materials (hydrogen, lithium and two-layer capacitor) and so on.The second chapter mainly discussed the different methods to prepare carbon nanotubes with chemical vapor deposition with different conditions and catalysts. The influence of the different catalysts and carriers on the structure and the appearance of the as-prepared carbon nanotubes was discussed, which showed that the catalysts and carriers are very important for the growth of carbon nanotubes prepared with chemical vapor deposition. With the bimetallic catalysts or the addition of the rare metal (Mo) to modify the content of the catalyst, the yield and the purity of the carbon nanotubes had been improved greatly. The diameter of carbon nanotube could be controlled to some extent by the adjustment of the ratio of the metallic catalysts and the carriers, and then single-walled carbon nanotubes was prepared by this way with the chemical vapor deposition method. In order to control the morphology and structure of the as-prepared carbon nanotube, the different carriers included the silicon substrate and the SBA-15 mesoporous molecular sieves were prepared and used to produce carbon nanotubes, the carbon nanotube arrays and the opened and straight carbon nanotubes were obtained.The third chapter studied the technology to produce large quantity single-walled carbon nanotubes with arc discharge method and the purification of the as-prepared single-walled carbon nanotubes produced with arc discharge method. In order to improve the yield and the purity of the single-walled carbon nanotubes, the technology and conditions were discussed and the traditional arc discharge method was improved to some extent. For the traditional method, a large amount of anode graphite deposits on the surface of the cathode, which decreases the utilized coefficient of the composite anode graphite. The arc discharge at the fixed angle could reduce the deposition on the cathode to some extent, and the deposition could be avoided when the circulating arc discharge was introduced between the anode and the cathode. With the above methods, the deposition containing the single-walled carbon nanotubes mostly present at the inner wall of the discharge apparatus, then purity and the quantity of the single-walled carbon nanotubes both improve greatly. In the following section, the purification of the single-walled carbon nanotubes produced with arc discharge method was studied. Following and improving the international purified method, with the procedure including the extraction, acidification, filtration and sinter, could obtain the single-walled carbon nanotubes with the purity about 95%. But the productivity of single-walled carbon nanotubes is relatively low, the research in this field need to be further studied.The fourth chapter studied the mechanical properties of the carbon nanotubes composite materials, which included the carbon nanotubes-Ni-P composite materials and carbon nanotubes epoxide-resin composite materials. Due to the unique mechanical properties and the one-dimension nano-structure, such as the intensity, the toughness, self-lubrication and so on, carbon nanotubes could be used as the additions to improve all kinds of properties of traditional materials. For the Ni-P and the epoxide-resin materials, the test results showed that the intensity, the toughness and the wearablity of the carbon nanotubes composite materials are obviously enhanced to some extent. It should be mentioned that the dispersion of the carbon nanotubes in the composite materials is very important, which influences all the properties of the nano composite materials.The fifth chapter theoretically calculated the field enhancement factor of the field emission from carbon nanotubes with the image floating sphere model and numerically simulated the field emission with the finite difference method by the self-edited program, and then the field emission from carbon nanotubes could be optimized in the fixed conditions. With the two methods, we got the similar conclusion, which could accord with the experiment result. The intertube distance of the carbon nanotube arrays critically influences the performance of the field emission from carbon nanotubes. Taking the field emission current density into account, the field emission from carbon nanotube arrays can be optimized when the intertube distance is 1-2 times of the height of carbon nanotubes. The calculated and the simulated results show that the anode-cathode distance slightly influences the field enhancement factor of carbon nanotubes. Even so, the threshold voltage for the field emission and the work voltage could be lowered to some extent. Comparing the two methods, the image floating sphere model is thought to be suitable to solve the problem of field emission from individual carbon nanotube and carbon nanotube array.