Experimental Research On Producing Carbon Nano-materials By Arc Discharge In Liquid And Its Magnetic Field Control

Carbon nano-material is one of the most active branches in nano technology research, and its potential application prospect is quite broad and attractive. Therefore, the fabrication process and formation mechanism of carbon nano-materials have been widely investigated. Thereinto, it is possible that arc discharge in liquid becomes an effective method for low-cost mass production of carbon nano-materials without sealed chamber, cooling device and vacuum system. To achieve this objective, in-depth study is needed on the growth mechanism of carbon nano-materials by arc dicharge in liquid and the effect of various arc parameters and external magnetic fields. Therefore, many experiments were conducted in this dissertation.At first, an experimental apparatus was designed for arc discharge in water. A series of experiments with different current values ascertained the advantageous current range to produce carbon nano-materials (25~120A). The solid products were observed and analyzed by scanning electron microscope and high-resolution transmission electron microscope. There are many multi-walled carbon nanotubes (MWCNTs), which were accumulated by the directional caron particles evaporated from anode, in the cathode depositions. The main structures in floating products are carbon nano-onions and irregular polygon particles, and they grew from isotropic carbon particles near gas bubble wall. The gas chromatography shows that the main chemical reaction is C+H2O=CO+H2, and it provides experimental basis for the theoretical analysis of plasma region. Through the research of arc discharge in water, the main factors to affect the formation of carbon nano-materials have been found, and they are the value and direction of arc current, stability of arc plasma, chemical character and cooling effect of discharge medium, shape of cathode surface, etc. To concretely study the effect of the factors in the carbon nano-materials formation, a series of carbon arc discharge under different conditions were conducted, such as alternation current, catalyst, liquid nitrogen and conical cathode. Many nano-structures were found including MWCNTs, carbon nano-onions, agglomerates of single-walled carbon nanohorns (SWCNHs) and carbon wrapped cobalt nano-particles. Their formation courses were analyzed respectively, and the roles of various factors were ascertained. describe the formation of carbon nano-materials by arc discharge in liquid. The model can be described as follows. During the steady arc discharge course, liquid is heated up by the hot arc plasma, so a gas bubble is formed around the arc column and afford an gaseous environment for arc discharge. Large number of carbon particles are evaporated from the graphite anode. Some of the particles react with liquid and form active gases. And other particles grow to carbon nano-structures. Thereinto, the directional particles move to the cold cathode surface and grow to MWCNTs along the arc axis, while the isotropic particles move to the gas bubble wall and grow to spherical carbon nano-particles. The active gases in the bubble may affect the formation of carbon nano-materials and corrode them.The model interpreted the formation mechanism of various carbon nano-structures by arc discharge in liquid, and afforded a theoretic basis for farther experimental research.As shown in the physical model, the stability of arc plasma and the directionality of carbon particles played important roles in the formation of carbon nano-structures. To study their effection, two magnetic fields were introduced to arc discharge in liquid respectively. The transverse alternating magnetic field (TAMF) disturbed the directionality of carbon particles, while the axial constant magnetic field (ACMF) strengthened it. Through the analysis of products, additional experimental support was afforded to the physical model. Some valuable new structures were found, and tentative theoretical analysis was put forward about their formation courses. Single-walled twist structures were obtained by carbon arc discharge with TAMF, and cylinder-shaped structures were obtained by carbon arc discharge with ACMF. Both of them have never been reported by other researchers at present. SWCNHs were firstly obtained in water with the help of TAMF. Thus, compared with laser ablation and arc in liquid nitrogen, it is hopeful to greatly depress the production cost. Furthermore, TAMF can increase the yield of spherical nano-paricles, such as carbon nano-onions and agglomerates of SWCNHs.In this dissertation, the foundation of the arc model improved the present production theory of carbon nano-materials by arc discharge in liquid. External magnetic fields were introduced to arc in liquid for the first time, in order to control the arc shape and the directionality of carbon particles. The primary control of carbon nano-products was realized, and some novel carbon nano-structures were obtained, such as single-walled twist structures and cylinder-shaped structures. This work is still blank in the domestic and international research fields, and it is very meaningful to explore.