Morphology of Nanoparticles and Fullerene Blacks Produced in a Carbon ARC
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JOHN HENRY J. SCOTT*, S. A. MAJETICH*#, AND S. DERRINGTON** *Department of Physics "**Departmentof Materials Science and Engineering Carnegie Mellon University, Pittsburgh, PA 15213 # Author to whom correspondence should be addressed.
ABSTRACT Using X-ray powder diffraction and transmission electron microscopy we examine the morphology and crystallographic structure of carbon-coated nanoparticles produced in a modified Huffman-Kratschmer carbon arc process. Graphite rods containing Ho were consumed in the arc and soots sampled from both the reactor walls and the inner core of the cathode deposit are compared. The inner core soot displays increased crystallinity and less amorphous carbon than wall-deposited soot, and nanoparticles in the inner core have more graphitic layers in the encapsulating shells, more gaps between the carbon coating and the core, and exhibit a greater degree of facetting than particles harvested from the reactor walls. These differences are interpreted as manifestations of the high temperature post-formation environment of the inner core of the cathode deposit. INTRODUCTION Since the discovery of carbon-coated nanoparticles by Ruoff and co-workers in 1993 [1], the field has aroused interest in several research groups already working with fullerene-derived materials. As the number of materials found to encapsulate grows, so does the richness of properties displayed by these species. To date, work has focused primarily on rare earth carbide [2-4] and transition metal [5] nanoparticles, but other metals and some alloys [6] have been studied. Novel properties displayed by these nanoencapsulates have led to their consideration for a wide variety of applications. The fine-particle magnetic behavior of these particles has drawn particular attention, and these materials are now being considered in areas traditionally dominated by iron oxides, such as magnetic toner for xerography, magnetic media for data storage, and ferrofluids for magnetic seals and bearings. Interest in these particles has even penetrated the biomedical community where it is hoped the carbon-coating may provide a biocompatible protective layer surrounding a magnetic core suitable for enhancing contrast in magnetic resonance imaging. A detailed understanding of the growth mechanism of encapsulated nanoparticles is critical if we hope to overcome the barriers to their exploitation and bring the applications mentioned above to fruition. Crystallographic structure and soot morphology provide important clues about how these particles form and are encapsulated in the electric arc process, but the large number of interrelated parameters affecting particle nucleation and growth in the reactor often confuse the issue. Extraction of reliable conclusions from morphological observations is often difficult. This study examines both rare earth and transition metal nanoparticles and attempts to isolate the effect of location of deposition within the reactor on nanoparticle morphology and structure. The morphology of soots recovered from Ho-cont
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