| Summary: | 碩士 === 國立臺灣大學 === 地質科學研究所 === 100 === Graphite encapsulated metal (GEM) nanoparticle is a spherical composite material with diameter ranging from 5-100 nm, the core is metal and the outer shell is comprised of graphite. The outer graphite shells protect the inner core metal nanocrystals against degradation reactions, preserving its inherited properties even when exposed to severe environments. Due to its unique properties, GEM has been widely applied, including in hydrogen storage and electromagnetic energy absorption. In the biomedical field, Fe-GEM has extensive applications, ranging from drug delivery agents to thermoseeds for hyperthermia therapy, mainly due to its excellent biocompatibility and magnetism. These results have shown that GEM is a versatile material with great potential in various fields.
The modified tungsten arc discharge method developed by Teng et al. and Dravid et al. in 1995 is definitely the most practical method, which routinely produces GEM in a relatively large quantity. Although it eliminates the ubiquitous problems confronting Krätschmer-Huffman arc synthesis of GEM, i.e., massive residual carbon debris, it suffers greatly from the unsatisfactory synthesis efficiency, which significantly hinders the progress of fundamental research and related applications of GEM. This poor synthesis efficiency was considered to result from the deficiency of evaporated carbon vapor, which could not be sufficiently concentrated since the solid carbon source could not uniformly dissolve and mix with metal during arcing.
Under the guidance of a two-step mechanism model, it is clear that the carbon-to-metal ratio within the coalescence region need to be adequately controlled. Thus, a liquid injection method was developed in this study, whereby a liquid carbon source jet directed at the arc could effectively increase the carbon concentration within the coalescence region, and therefore enhance the synthesis efficiency. The experimental results show that the synthesis efficiency of ferromagnetic GEM increased dramatically, i.e., for Co-GEM and Ni-GEM, it was raised from 20-30 wt% to around 80 wt%; for Fe-GEM, it was raised from less than 10 wt% to 42 wt%. Since iron possesses greater carbon solubility, a large portion of the dissolved carbon in iron forms stable carbide instead of encapsulation layers; therefore, the encapsulation efficiency of Fe-GEM is lower than that of Co-GEM and Ni-GEM.
The effect of carbon content on encapsulation efficiency was investigated by using methanol, ethanol and 1-propanol, as carbon sources to synthesize GEM. The experimental results show that the encapsulation efficiency apparently increases with increased carbon content, i.e., when adopting methanol, ethanol, and 1-propanol as carbon source, the encapsulation efficiencies were 27%, 80%, and 85% respectively. In addition, the size distribution of Co-GEM was surprisingly found to shift to a lower size value in response to the carbon content, with the average size reduced dramatically from 86 nm to 16 nm, attesting to the effectiveness of the liquid injection method in both raising the encapsulation efficiency as well as size control. Furthermore, by adopting 1-butanol as the carbon source, the average size of the as-received powders could be further reduced to around 12 nm.
Due to the significant difference between the evaporation rates of Cu and of graphite, it has been a challenge to synthesize Cu-GEM by a modified tungsten arc discharge setup. In this study, 1-propanol was adopted as the carbon source to synthesize Cu-GEM; the experimental results show that without lowering the evaporating quantities of copper, the carbon-to-metal ratio has been successfully manipulated by simply increasing the carbon vapor concentration within the coalescence region. Unsurprisingly, the encapsulation efficiency and production rate were dramatically increased; i.e., for the encapsulation efficiency, it was raised from merely 4 wt% to 40.6 wt%, while for production rate, it was increased from unsatisfactory 0.06 g/min to 0.7 g/min. In this study, we have demonstrated by experimental results that the newly developed method is effective not only in controlling the size of GEM, but also in increasing the encapsulation efficiency of both ferromagnetic GEM and GEM with a low m.p. core metal such as Cu-GEM.
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