Synthesis of Fe-filled carbon nanocapsules by an electric plasma discharge in an ultrasonic cavitation field of liquid e
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Nanoparticles of iron carbides (Fe3C and -Fe2.5C) wrapped in multilayered graphitic sheets were synthesized by a developed method in which an electric plasma was generated in an ultrasonic cavitation field containing thousands of tiny activated bubbles in liquid ethanol. Annealing changed the phase composition, structure, and size of the carbon nanocapsules as most of the iron carbides decomposed into the ␣-Fe phase and graphite. Powder samples annealed at 873 and 973 K have maximal saturation magnetization values equal to 80.6 and 83.4 A m2/kg, respectively, which is approximately 40% of the value of bulk iron. Using this method, it will be possible to synthesize nanoparticles of a metal of choice encapsulated by graphite shells by selecting appropriate materials for the ultrasonic tip and electrodes.
I. INTRODUCTION
There has been an increasing interest in recent years in the development of new nanoscale magnetic materials for recording media in computer technology,1 biomedical,2 and other applications. The magnetic nanoparticles of 3d metals (iron, nickel, and cobalt) coated with crystalline graphite shells, called “carbon nanocapsules,” are examples of such interesting materials. The coating helps to prevent the aggregation and environmental degradation of the metallic cores. To date, carbon-coated nanocapsules have been synthesized by a conventional arc discharge3 and a modified arc discharge4 in a gaseous atmosphere. Before undergoing arc vaporization, the metal (or metal oxide) precursors are normally packed inside a cavity drilled into a graphite electrode. However, this method requires expensive vacuum equipment and high electric power for the generation of the arc discharge. A synthetic method for producing iron carbide nanoparticles wrapped in multilayered graphitic shells has been demonstrated previously in an experiment using liquid ethanol.5 This method takes advantage of the fact that an electric plasma can be generated and maintained
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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2006.0316 2524 J. Mater. Res., Vol. 21, No. 10, Oct 2006 http://journals.cambridge.org Downloaded: 25 Mar 2015
in an organic liquid under ultrasonic irradiation. Ultrasonic cavitation causes a very highly localized high temperature and pressure region where tiny bubbles are collapsing—a region referred to as a so-called “hot spot.”6–8 An ultrasonic cavitation field, with its many activated tiny bubbles, enhances electrical conductivity because of the radicals and free electrons formed within it. Thus, an electric plasma discharge can be generated at a remarkably low voltage, such as 55 V DC, even in insulating organic liquids such as benzene and ethanol.5,9,10 Without the ultrasonic irradiation, no plasma discharge can take place at such low electric power levels. Usually, a transient high voltage is necessary to generate the plasma discharge in a neutral medium. Breakdown voltages for the generation of plasma discharge in gaseous phases such as air, oxygen, and n
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