| Summary: | 碩士 === 國立臺灣大學 === 地質科學研究所 === 104 === Graphite Encapsulated Metal (GEM) nanoparticles are spherical core-shell structured composite material with a diameter ranging from 5–100 nm. The core of GEM is metal, and its outer shell is composed of several layers of graphite/graphene which can preserve the inner core in a severe environment, such as from acid erosion and oxidation. It is well known that many different functional groups, including carboxyl and hydroxyl, can be easily attached to the surface of carbon materials. Recently, several studies have revealed that GEM has a great potential to become a novel material including, for example, in hydrogen storage and biomedical materials due to its unique properties. For instance, Wu et al.(2007) used polyethylene glycol and folic acid grafted on Fe-GEM for the heat treatment of cancer, and Chung et al. (2009) used Co-GEM as an electrochemical hydrogenation material.
The modified tungsten arc-discharge method was developed by Teng et al. and Dravid et al. in 1995. This is the most practical method for producing a large quantity of GEM because it reduces the amount of carbon debris origin compared to the Krätschmer–Huffman method. However, the encapsulation efficiency of GEM remains low. Until 2012, with the help of the two-step mechanism model, we used n-propanol as the liquid carbon source to synthesize GEM, significantly increasing its encapsulation efficiency from 20–30 wt% to around 80 wt%, and presenting a preliminary method for controlling the particle size of GEM through different liquid carbon sources.
However, we faced two difficult issues after switching the carbon source from solid to liquid. First, this method could disturb the arc discharge which causes the discontinuity of the experiment, leading to the lockout unsustainable injection. Second, the consumption rate of the tungsten rod rose from 1 mm/h to 420 mm/h, making it difficult to synthesize large quantities of well-encapsulated GEM. In addition, the detailed mechanism, after entering the liquid carbon source, still remains unclear.
The purposes of this study are to realize the changes of arc in the cabin and to resolve problems after using the liquid carbon source. In order to solve the problems, this research has installed a liquid metering pump to regulate the amount and direction of each injection, so that the carbon source can be directed to mainly spray the synthetic region of GEM, which is called the coalescence region. This method avoids the resistance caused by dripping liquid along the tungsten rod, and successfully sustained the experiment. The TEM images show that the synthesized GEMs, using a liquid metering pump, retain a complete core-shell structure, and the utilization of carbon source calculated by TGA data shows significant improvement, from 20% to 64%.
Furthermore, we listed the possible reasons causing the high consumption rate of tungsten rod, and verified them by theoretical calculation and manner of experiments, one by one. It can be speculated by OES data that the dominant gas in the center of the arc changed from helium to hydrogen. In the meantime, the arc temperature rose show by the color changing into blue and white, representing the higher arc temperature is the main reason causing the tungsten melting rate to increase 420-fold. After calculating the heat conduction, we confirmed that increasing the diameter of the tungsten rod can immediately solve this problem.
Since it is feasible to control the injection rate through the use of a liquid metering pump, we tried to figure out the encapsulation efficiency of GEM over time when synthesizing GEM. Under the same experimental parameters and total liquid injection volume, we compared the results of two different injection rates, 10μL/min and 100μL/min, and found that using the former injection rate can result in 5-fold higher encapsulation efficiency. According to the two-step mechanism of GEM, we speculated that adding liquid carbon source during arc discharge would rapidly increase the carbon proportionate of the coalescence region; however, the carbon vapor will quickly leave the coalescence region via convection. Thus, for the same total liquid injection volume, taking a small amount and injecting it a few times is the best way to inject the liquid carbon source; it can significantly improve the encapsulation efficiency and the utilization rate of the carbon source.
Lastly, based on the experiment results, we proposed a model that can explain the transformation of the arc body from bell-shaped to columnar, after injecting the liquid carbon sources. Furthermore, our model raises the potential of employing GEM to fundamental science and applied material fields.
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