Application of methane and helium gas mixture in the synthesis of graphite encapsulated nano nickel grains
Helium, methane
Graphite wrapped nanocrystals are spherical composite materials with particle sizes ranging from 1 to 100 nanometers (nm), with a metal core and graphite outer layer. It was first discovered in experiments using the Kr ä tschmer Huffman method to manufacture derived materials of carbon-60 in 1993 (Tomita et al., 1993, Ruoff et al., 1993). At that time, many people wanted to insert other metals into the vacancies in the middle of carbon-60, but unexpectedly found a small amount of graphite wrapped around nanocrystals in the experimental product. However, the amount of products produced by this method is extremely small (about a few hundred particles), making it impossible to conduct basic scientific research. In 1995, Teng et al. and Dravid et al. proposed a new method of using tungsten electrodes as cathodes and graphite crucibles as anodes, and arranging the electrodes vertically to replace the old method of horizontal configuration. The advantages of this improved tungsten arc method are: high stability, long experimental time, and less impurities generated; However, due to the high proportion of metal, the source of carbon that can be used for encapsulation is relatively insufficient, making it difficult to efficiently encapsulate all metal particles, resulting in only a small amount of pure graphite encapsulated nanocrystals being recovered after purification and separation of the product. Currently, scientists have attempted to add a small amount of methane to helium gas to increase its impact on recovery rate.
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For the problem of insufficient carbon content, Lin Chunchang (2002) proposed adding a vacuum heat treatment program before the purification and separation steps, so that the metal particles of the initial product can be more completely coated with graphite, thereby reducing carbon impurities. This study attempts another method to increase carbon content by adding methane gas to the tungsten arc process. Through the decomposition of methane, it provides a carbon source other than graphite raw materials to solve the problem of insufficient carbon content.
The experimental results showed that adding a small amount of methane to high-purity helium gas in Beijing had a significant effect on increasing the recovery rate; Under low current conditions, the recovery rate can reach about 60%, while under high current conditions, it is also about 30%. Compared to pure helium experiments, the recovery rate is less than 5% for both high and low currents. There are three possible reasons why methane can improve the recovery rate:
Firstly, the carbon produced by the decomposition of methane at high temperatures, through the catalytic effect in a two-step mechanism, can make the outer layer of incompletely wrapped particles more complete.
Secondly, the hydrogen atoms produced by methane decomposition combine near the arc zone, releasing heat that helps maintain high temperatures near the arc zone.
Thirdly, hydrogen atoms decomposed by methane can be attached to dangling bonds at the edges of graphite, which helps with the decomposition of graphite. For graphite raw materials initially added to the crucible, they can be dispersed in smaller pieces in the metal, thus facilitating the evaporation of graphite. As for the lower recovery rate under high current conditions, it is speculated that the reason is that the amount of metal vapor is higher than that under low current conditions, but the amount of carbon vapor does not increase relatively. For the total production, the experimental results of high current transformation with the addition of methane are better, but due to its poor recovery rate, this experimental condition combination is prone to waste of nickel metal raw materials. If the device is improved to continuously circulate methane/helium in an appropriate ratio during the experiment, it should be possible to improve the recovery rate of high current experimental products.
