Graphite encapsulated nanocrystals produced using a low carbon : metal ratio
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Graphite encapsulated nanocrystals produced using a low carbon : metal ratio Jonathon J. Host, Mao H. Teng, Brian R. Elliott, Jin-Ha Hwang, Thomas O. Mason, D. Lynn Johnson, and Vinayak P. Dravid Department of Materials Science and Engineering & Materials Research Center, Northwestern University, Evanston, Illinois 60208 (Received 29 April 1996; accepted 16 December 1996)
Graphite encapsulated nanocrystals produced by a low carbon tungsten arc were analyzed to determine their chemistry, crystallography, and nanostructural morphology. Metallic nanocrystals of Fe, Co, and Ni are in the face-centered cubic (fcc) phase, and no trace of the bulk equilibrium phases of body-centered cubic (Fe) and hexagonal close-packed (Co) were found. Various analytical techniques have revealed that the encased nanocrystals are pure metal (some carbide was found in the case of Fe), ferromagnetic, and generally spherical. The nanocrystals are protected by turbostratic graphite, regardless of the size of the nanocrystals. The turbostratic graphite coating is usually made up of between 2 and 10 layers. No trace of any unwanted elements (e.g., oxygen) was found. The low carbon : metal ratio arc technique is a relatively clean process for the production of graphite encapsulated nanocrystals.
I. INTRODUCTION
The synthesis of buckyballs and various other hollow cage fullerene derivatives1 led to speculation that various materials could be incorporated into large fullerenes, thus protecting this material from environmental effects. This was shown to be possible for carbides2 by simply adding some metal3 or metal oxide4 to the anode in a graphite-graphite arc, based on the Kratschmer –Huffman1 carbon arc technique. The relative ease of carbide encapsulation using the “stuffed anode” method5 resulted in the encapsulation of many carbides, including BC,6 various transition metal carbides such as Sc3 C4 7 and ZrC,8 and many rare earth carbides, ranging from LaC2 to LuC2 .2,9–11 The successful encapsulation of carbides suggested the possibility that pure metals could be encapsulated. Encapsulated ferromagnetic metals should have interesting properties and commercially valuable applications, ranging from magnetic data storage and ferrofluids to biomedical applications,12 due to their retained magnetic properties and immunity to most environments as a result of the protective graphite coating. Graphite-encapsulated metal nanocrystals (GEM nanocrystals)13 of Fe,14 Co,15 Ni,16 Cu,17 Sm–Co,18 and FeNdB19 have been produced in small quantities, but with a large amount of carbonaceous debris. It has been difficult to synthesize GEM nanocrystals using the Kratschmer–Huffman process due to the ease of carbide formation, and the fact that Fe,20 Ni,21 Co22 (and some carbides23 ) sometimes catalyze carbon nanotubes or “sea urchin” structures24 (where carbon nanotubes grow radially from the metal nanocrystal) instead of graphite layers. The production of large amounts of commercially 1268
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