Electronic Thesis/Dissertation

 

Synthesis and Diagnostic of Single and Multi-dimensional Nanomaterial in Arc Discharge Plasma Open Access

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Anodic arc discharge plasma is an environmental friendly and effective method to synthesize carbon nanostructures. Some of the nanostructures such as Multi-Walled Carbon Nanotubes (MWCNTs), Single-Walled Carbon Nanotubes (SWCNTs), Carbon Fullerenes and Boron Nitride Nanotubes (BNNTs) were first found using arc discharge plasma. Due to the high temperature synthesis environment, the products synthesized in arc discharge have less defects and better crystal structures. 1 Dimensional structure such as MWCNTs and SWCNTs have been very well studied in the previous research. Now, we would like to focus on synthesizing 2D structures such as Graphene, and 3D Carbon Encapsulated Magnetic Nanoparticles (CEMNs). In this dissertation, Graphene Platelet Networks (GPNs), comprised of randomly oriented 2-3 atomic layer thick graphene flakes, are synthesized using a novel plasma-based approach. The approach uses a substrate capable to withstand synthesis temperatures around 800 ℃, but is fully independent of the substrate material. The synthesis occurs directly on the substrates’ surface without the necessity of any additional steps. GPNs were synthesized on various substrate materials including silicon (Si), thermally oxidized Si (SiO2), molybdenum (Mo), nickel (Ni) and copper (Cu), nickel-chromium (NiCr) alloy and alumina ceramics (Al2O3). Later on, a proof has been given that the carbon flux from the arc along with a particular substrate temperature is the key factor of the GPNs synthesis. And then the GPNs grown on the silicon substrate have also been destroyed by different methods including oxidation, glow discharge, overheating the substrate during the synthesis procedure, ion sputtering and plasma coating. The substrates were then put back to the chamber to regrow the GPNs on top of the destroyed surface. This experiment proves that plasma has a healing effect to regenerate the material that was consumed by plasma or any other type of hazard conditions. Carbon Encapsulated Magnetic Nanoparticles (CEMNs) have been successfully synthesized using arc discharge plasma equipment. By adding an external magnetic field, the size distribution of the CEMNs is controllable. With the influence of the 0.06T single magnetic field and 0.08T parallel magnetic field, we can successfully synthesize uniform crystal lattice CEMNs with a size distribution of 20~60 nm and 10~50 nm respectively. This result could be applied to Fe, Ni, and Fe+ Ni CEMNs synthesis. The cytotoxicity of these magnetic nanoparticles has been tested with the Human breast adenocarcinoma cell line MDA-MB-231 in order to further investigate the biomedical applications of these CEMNs. We then found out the best concentration, when applied to MDA-MB-231, is 0.0001 – 100 ug/ml. To better understand the synthesis mechanism of forming the nanostructure in arc discharge plasma, two in-situ diagnostic methods have been used in this dissertation. Localization of the SWCNTs synthesis in arc discharge plasma volume in situ has been a long-standing problem. This relates to the ability of controlling volumetric synthesis of nanostructures in plasmas in general. We have developed an actuator driven high speed probing system, which is able to extract material from the arc plasma volume during the synthesis procedure. It is shown that the growth region of SWCNTs is between 3mm to 11mm away from the center of the arc discharge. Depending on the origin, the length of the SWCNTs increases non-monotonically up to 500 nm. The other in-situ diagnostic method is Langmuir probe. It was used to measure Carbon and Molybdenum ion density along the horizontal direction from the center to the arc plasma under 0.15 Torr, 10 Torr and 500 Torr respectively. The ion current density distribution results for both Carbon and Molybdenum were then found to be relatively close to the simulation results provided by Tech-X. Further experiments also proved that under different background gas pressures, the carbon structure synthesized at the same carbon ion density positions and on the same substrate temperature can be different (either GPNs or amorphous carbon). So it is also proved from the aspect of the ion current density that GPNs can only form in a near vacuum background gas pressure and on a heated substrate of about 800 degree Celsius. The results obtained from this dissertation are significant in providing more controllable and efficient methods synthesizing GPNs and CEMNs. And two in-situ diagnostic systems, namely fast probing system and Langmuir probe, have provided new information in understanding the mechanism during the nanomaterial synthesis in arc discharge plasma. Results presented in this dissertation were published in 2 scientific papers and two more papers are under preparation.

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