| Summary: | 碩士 === 國立臺灣大學 === 地質科學研究所 === 90 === Graphite encapsulated nanoparticle is a new composite material with a core/shell structure. The core may be pure elements or carbides, and the shell is graphite or amorphous carbon. This new material has great potential in various applications, not only because its nanocrystalline spherical structure, but also because the shell can protect the core material from sever environmental reactions such as oxidation, hydrolysis, or acid erosion.
Graphite encapsulated nanoparticles were first found in 1993 by two groups, one is Saito et al. in Japan, and the other is Subramoney et al. in USA. They used Krätschmer-Huffman method to synthesize the particles, but the very low production rate and the high percentage carbonaceous debris were the two major problems of this method. To improve the Krätschmer-Huffman method, Dravid et al. developed a new tungsten arc method in 1995. The new method uses tungsten rods as cathode, and graphite crucible with metal source materials as anode. The tungsten arc method raises the production rate and greatly decreases the amount of carbonaceous debris in the product, however, it is still suffer from low recovery ratio after acid wash procedure due to relatively low carbon content.
To increase the recovery ratio of the initial product, Lin (2002) has demonstrated that a heat treatment process prior to the acid bath procedure can effectively increase the recovery ratio. In this research, we introduce some methane into the chamber atmosphere during the arcing process, so as to provide additional carbon sources that may facilitate the encapsulation process of the particles.
The results show that the new methane procedure can greatly increase the recovery ratio of initial products. The ratio has been raised to 30~60%, comparing to less then 5% if we use helium only. There are three possible reasons for this result: 1. the carbon coming from the decomposing methane may provide additional carbon sources that makes the particles to be encapsulated easier. 2. some of the hydrogen atoms coming form the decomposing methane may reunite into molecules after passing through the arc, and therefore may release additional heat to maintain the required high temperature around the arc, effectively prolong the reaction time of the particles. 3. the hydrogen atoms may also help to etch the graphite into small pieces of polycyclic aromatic hydrocarbon and be separated in the melting nickel pool more easily.
The equipment now can only take up to 40 Torr methane with 200 Torr helium atmosphere in order to have a steady arc. If we can introduce low ratio CH4/He gas continuously during the experiment, the recovery ratio can be higher. In the future, we’ll set up a continuous flow of methane/helium mixed gas with various mixing ratios and total pressures. Hopefully the new setup can greatly increase the recovery ratio of the desired nanoparticles.
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