| Summary: | 碩士 === 國立臺灣大學 === 地質科學研究所 === 94 === Graphite Encapsulated Metal (GEM) nanoparticle is a new spherical composite material with a diameter ranging between 5 and 100 nm. It has a core/shell structure, where the core is metal and the shell is graphite. Having the special structure and nanosizes, GEM has become an interesting research subject for the academic community.
For the last several years, we have been using a tungsten arc-discharge method to synthesize GEM, and have successfully increased the yield and the recovery ratio of the GEM. However, we still know very little about the changes of the raw materials inside the crucible during the runs. The lack of knowledge could be a barrier keeping us from both to improve the synthesis process and to develop a mathematic model for the system. To fill up the gap, this study specifically designed to investigate what has been happening to the raw materials in the graphite crucible.
The work can be divided into two parts: First is to design and do a number of synthesis experiments that will produce suitable samples for the later analysis. Second is to analyze the metal blocks left in the crucible after the synthesis experiments. The metal blocks were first cut, polished and observed under the microscope; finally, the blocks were put into an acid-bath and fully dissolved. Only the insoluble graphite flakes stayed intact in the solution for the size analysis.
The morphology on the upper surface of the metal block is closely related to the arc-discharge during the runs. The center smooth and shining area directly contacts with the arc plasma, while the surrounding rough and foggy area, on which the carbon and metal vapor condensed and deposited, stays outside of the arc. Chemical analysis shows both areas are covered with carbon (i.e., graphite.) In addition, many radial oriented small metal blobs can be found at the rough area. Many graphite flakes existed inside the metal blocks, and their distribution and shapes are related to the temperature gradient caused by the arc-discharge. The temperature at the center portion of the block is the highest, therefore only smaller pieces and less amount of graphite can be found. The temperature at the bottom and wall portions is relatively lower, where many larger graphite flakes can be found. The log-normal size distribution of the graphite flakes in the metal blocks indicates the graphite formed by a nucleation and growth process.
Using diamond carbon source has greatly improved the synthesis efficiency. Evidence shows that small graphite flakes were formed and come off from the surface of the diamond powder. The existence of these small graphite flakes has significantly increased the contact area between graphite and metal, thus increased the amount of carbon dissolved into the metal. When evaporated from the melting metal pool, the carbon and metal vapor mixed more uniformly, and therefore synthesize more well-encapsulated GEM.
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