Synthesis of carbon-encapsulated magnetic nanoparticles by a grain-boundary-reaction
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Synthesis of carbon-encapsulated magnetic nanoparticles by a grain-boundary-reaction Qixiang Wang, Guoqing Ning, Fei Wei, and Guohua Luo Department of Chemical Engineering, Tsinghua University, Beijing 100084, China. ABSTRACT Carbon-encapsulated ferric magnetic nanoparticles were prepared by the grain-boundary-reaction of ultrafine goethite particles. The mechanism of the grain-boundary-reaction was studied with high-resolution transmission electronic microscope, X-ray diffraction and thermo gravimetric analysis. The magnetic properties are measured with a vibrating sample magnetometer. The diameter of carbon-encapsulated ferric magnetic nanoparticles is 30~60 nm, and the coercive force and saturate magnetization are 315 Oe and 30 emu /(g powder), respectively. These composite particles are very stable in air. INTRODUCTION Carbon-encapsulated ultrafine metal particles, with particle diameters less than 100 nm, exhibit properties which make them valuable for many applications, including their use as novel catalysts, sensors, membranes, structural, electric, magnetic, and biomaterials [1-3]. The high temperature catalytic reduction of carbon monoxide on the surface of iron particles is a well-known method for making ultrafine carbon-coated iron particles. However, the most successful method for making carbon-encapsulated ultrafine metal particles with an improved formation rate is the high current carbon arc evaporation process [2]. These techniques however, are not suitable for mass production because the rates of particle formation and production are slow. Ultrafine particles of acicular goethite, hematite and iron metal, are known to exist as polycrystals with the size of individual crystallites being in the order of several (1~10) nm [4]. They consist of the following two components: a crystalline component where the atoms are located in the lattice of the crystallites (grain) and an interfacial component comprising atoms which are situated in the grain boundaries between the crystallites [5]. Chemically, the interfacial and surface atoms of the nanocrystallines are more active than those of the grains [6,7]. In this paper, we report the production of carbon encapsulation of ferric magnetic nanoparticles by using a particle grain-boundary-reaction with the starting materials of ultrafine goethite particles and ethylene. Goethite particles are abundant resources in nature, and this method gives a higher yield and would be a commercially viable (effective and continuous production at a low cost) process in a packed bed reactor or a fluidized bed reactor.
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EXPERIMENTAL The carbon encapsulated magnetic nanoparticles were grown by passing carbon-containing gases (C2H4) at elevated temperatures over acicular goethite particles. The goethite particles wer
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