在電弧系統中以複合碳源合成石墨包裹金屬奈米顆粒之機制探討

Abstract

石墨包裹金屬奈米顆粒(graphite encapsulated metal nanoparticles, GEM)為一種核殼奈米複合材料，其內核為金屬，外殼為石墨層，其穩定的石墨層耐酸蝕，抗氧化，能保護內核金屬不受環境影響而生存在極端環境，使被包裹之金屬可以展現其特性而被應用在諸多領域，其中，以石墨包裹鐵奈米顆粒能應用於生醫領域、微波吸收以及環境工程等等，其應用領域最廣，然其良率與產率卻未及石墨包裹鎳奈米顆粒。 1997年Elliott等人提出二步驟機制(Two-Step Mechanism)為目前較能合理解釋以電弧法合成GEM顆粒之機制，而隨碳源之改良，石墨包裹金屬奈米顆粒之良率亦也提升，其中苯蒸氣法為能提升包裹良率亦不易斷弧之方式，然良率達80%後卻遇到瓶頸，因此本研究為提升良率，分析鐵金屬特性，及苯蒸氣法影響良率之因素，將合成石墨包裹金屬奈米顆粒之合併區分為高溫合併區與低溫合併區，推論苯蒸氣從低溫合併區向高溫合併區擴散時，碳蒸氣有逐漸下降而不足之現象。因此為提升碳蒸氣量，使用複合碳源，苯蒸氣法，及沸點低且容易熱裂解的固態碳源萘於坩堝，當電弧撞擊時，受高溫熱裂解之萘能從高溫合併區向低溫合併區，能提升合併區之碳蒸氣量，分別使內核為鐵與鎳之顆粒的良率提升至88%及92%，突破良率之侷限。 而隨GEM的使用率的增加，內核為非鐵磁性金屬及合金之顆粒貢獻亦也增加，本研究利用複合碳源製備成功首例以電弧法合成石墨包裹鋁奈米顆粒，突破容易形成碳化物之瓶頸，為研究上的一大突破，其次，合成內核為鐵鎳、鋁鎳合金之顆粒，顯示出鎳之優良催化效果能提升顆粒之包裹良率。 最後，以複合碳源苯與萘建立模型，建立碳蒸氣濃度在合併區之分布，並將合併區分為高溫合併區與低溫合併區，使顆粒產生石墨層之方式，前者以相分離為主，後者以催化機制為主，其中催化機制分為二步驟機制之催化，及金屬催化碳氫化合物在金屬表面上斷鍵並產生石墨。

Graphite Encapsulated Metal (GEM) nanoparticles are core-shell composite-structured nano-materials with an inner core of metal and outer shells of graphite layers. Because of the protection of the stable graphite shell which can resist against acid erosion and oxidation, the internal metal can be preserved in server environment and exhibited its characteristics. It has wide application especially for graphite encapsulated iron (Fe-GEM) nanoparticles, for example, wastewater treatment, bio-technology and microwave absorber. However, the production rate and encapsulation efficiency of Fe-GEM is not as high as graphite encapsulated nickel (Ni-GEM) nanoparticles. In 1997 years, Elliott et al., proposed the most fundamental mechanism, Two-Step Mechanism, to explain the detail synthesis process. The encapsulation efficiency and production rate of GEM is affected by the sufficient and uniform carbon vapor in the coalescence area. However, to reach good vapor environment is not easy because of metal vapor, catalysis ability and metal carbide which decisively influences the result. From the result of Encapsulation efficiency of Fe-GEM and Ni-GEM, discussing the factor of iron and limitation of benzene is necessary that benzene can’t diffuse into high temperature coalescence area because of high molecular weight and air convection. To improve the carbon vapor distribution, adding the solid carbon resource, naphthalene, in crucible to enhance the carbon vapor in high temperature coalescence area is valid to raise encapsulation efficiency to 88% and 92% for inner core of iron and nickel respectively. Also, graphite encapsulated aluminum nanoparticles which is support unable to produce because of carbide is successful made. It shows the necessary of excess and uniform carbon vapor in metal vapor. According to the result. It is supported to establish the forming graphite or carbon soot of high temperature coalescence area, phase segregation domain, and low temperature coalescence area, catalytic domain. This procedure is expected to establish a preliminary concept to provide the foundation for more in-depth materials science research in the future.