Preparation, structure, and magnetic properties of cobalt nanoparticles in carbon fibers
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T. Zabel, B. Wiedenhorst, and H. Micklitz II. Physical Institute, University of Koeln, Zuelpicherstrasse 77, 50937, Koeln, Germany (Received 25 January 2001; accepted 19 July 2001)
Samples of Co clusters embedded in a carbon fiber matrix have been prepared using a heat-treatment method of cellulose fibers with ion-absorbed cobalt cations. Depending on the heat-treatment temperature used, different cluster sizes have been obtained, ranging from superparamagnetic clusters (average size 10 nm) to ferromagnetic nanoparticles (size between 30 and 200 nm). The formation of graphite planes surrounding the nanoparticles has been observed. I.
INTRODUCTION
Nanoparticles are a bridge between the bulk solids and the atoms. They have a significant size dependence of electrical, optical, magnetic, and chemical properties. Granular solids composed of magnetic nanoparticles dispersed into nonmagnetic matrixes exhibit a wide variety of interesting magnetic and electronic properties that can be engineered by proper choice of the composition. Among these properties the following can be named: a size-dependent enhanced magnetic moment;1 giant magnetoresistance;2,3 tunneling magnetoresistance.4,5 These granular solids are usually fabricated by codeposition,6 sequential deposition of thin metallic layers and thicker insulating layers,7 sol-gel method,8 or the combination of ion-beam metal cluster preparation with the inert-gas matrix-isolation technique.9 There is a potential need for methods for producing bulk amounts of magnetic particles embedded into a nonmagnetic matrix, e.g., surrounded by closed graphitic shells, in a high-yield process. The possibility of variation of the resistance of chemically modified carbon fibers with additives in a wide range allows to one study the electrophysical properties of magnetic and nonmagnetic metallic clusters in an insulating, semiconducting, or metallic carbon matrix. II. EXPERIMENTAL
To obtain cobalt clusters embedded in a carbon matrix the heat-treatment method of carboxylated cellulose fibers after the exchange of COOH-group protons by cobalt cations by means of ion-exchange absorption has a) b)
Department of Chemistry. Department of Physics.
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J. Mater. Res., Vol. 16, No. 10, Oct 2001 Downloaded: 12 Apr 2015
been employed. The homogeneous distribution of cations on an atomic scale in carboxylated fibers as resulting from ion-exchange absorption gives the opportunity to obtain a homogeneous distribution of cobalt clusters within the carbon fibers after heat treatment. 3-carboxylcellulose (TCC) was used as a carboxylated cellulose. It was obtained on a base of rayon fibers by means of introducing of carboxyl groups in accordance with the following equations:10 [C6H7O2(OH)3]n + nHIO4 → [C4H5O2(OH)(CHO)2]n + nHIO3 + nH2O , (1) [C4H5O2(OH)(CHO)2]n + 2nN2O4 → [C3H3O2(COOH)3]n + 4nNO + nH2 .
(2)
The oxidation of cellulose by periodic acid in accordance with Eq. (1) was carried out within 5 h at 20 °C. The concentration of periodic acid in the solution
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