Formation of Nanoparticles in a Carbon ARC
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Here we bring together existing experimental evidence and new results to postulate the details of carbon-coated nanoparticle formation. We focus on basic questions concerning nanoparticle growth, concentrating on the characteristics which make them desirable for various applications. The main advantages of nanoparticles made by the carbon arc process are: 1) their resistance to oxidation and hydrolysis, 6 2) the abundance of particles in the 5-50 nm size range, 3) the ability to generate alloy nanoparticles, 7 and 4) the ability to prepare high temperature metastable phases. 8 The first of these advantages exists due to the carbon coating, the second depends on the details of cluster growth, the third arises from the microscopic homogeneity of the carbon arc, and the fourth due to the rapid quenching rate. Understanding the formation mechanism is therefore critical for engineering nanoparticles with desirable properties. EXPERIMENTAL The evidence for nanoparticle formation is obtained by studying the product morphology and relating it to different experimental parameters, rather than from direct examination of the growth process. Our understanding of the particle morphology arises mainly from transmission electron microscopy (TEM) images of the nanoparticles, which indicates their size, shape, and crystallinity. In addition, electron microdiffraction and x-ray powder diffraction yield
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information concerning the nanoparticle crystalline phases. Finally, energy dispersive spectroscopy (EDS) reveals the elements present. Reactor parameters investigated here include the materials the particles are made from, the buffer gas pressure, and the temperature of the surface where the particles deposit. The bulk of data concerns either rare earth carbides or transition metals. The composite anode rods used in the carbon arc are either pure graphite drilled and packed with metal or metal oxide powders, or mixtures of graphite and metal or metal oxide powders formed into cylindrical rods, typically of 1/4 in. diameter. In some cases organic binders were used to solidify the powders. The grain size of the powder starting materials is on the order of tens of microns for most materials, and is never in the nanoparticle size regime. The typical conditions for preparing nanoparticles range from those optimized for the fullerenes (100 A DC current, 125 Torr He) to the higher pressure (500 Torr He) conditions which favor carbon nanotubes. Normally the helium is flowing through the reactor, but we have also tested static burning (without He flow) for quantitative analysis of the reactor products. RESULTS AND DISCUSSION Nanoparticles are found in all parts of the reactor, both on the walls and in different regions of the cathode deposit, 9 demonstrating that the nanoparticles do not form only as liquid droplets on the high temperature cathode. 10 Metal and metal carbide clusters form along with the fullerenes in the high density plasma. Collisions with the He buf
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