Thermal Plasma Synthesis of Fe-Co Alloy Nanoparticles
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2 , KRISHNA CHOWDARY 3, MICHAEL E. JOHN HENRY J. SCOTT *, ZAFER TURGUT 3 2 MCHENRY , and SARA A. MAJETICH 1 National Institute of Standards and Technology (NIST), Gaithersburg, MD 20899. 2 Department of Materials Science and Engineering, 3Department of Physics, Carnegie Mellon University, Pittsburgh, PA 15213. * Author to whom correspondence should be addressed. email: [email protected]
ABSTRACT FeCo nanoparticles are synthesized in a radio frequency (RF) plasma torch from metal powder precursors and simple gases. The first precursor consists of pre-alloyed FeCo powder; the second precursor is a mixture of elemental Fe and Co powders. A protective carbon coating on the particles is achieved by injection of acetylene gas into the plasma. X-ray diffraction reveals phase purity, low carbon concentration, and minimal oxidation. Analytical electron microscopy is used to examine the nanocomposite morphology and composition of individual nanoparticles. Both synthesis routes produce alloy product, but nanoparticles produced from pre-alloyed precursors exhibit smaller variations in Fe/Co ratio than particles produced from elemental powders. INTRODUCTION
Magnetic nanocomposites often display interesting properties because of their nanometer scale structure and reduced dimensionality [1-3]. Magnetic nanoparticles are particularly interesting because of their potential for large-scale synthesis and technological applications. Magnetic nanoparticles are already employed as data storage media [4], ferrofluids, magnetic inks, xerographic toners [5], and biomedical imaging contrast agents. Additionally, powder metallurgical processing techniques such as compaction can transform raw nanoparticle products into bulk nanocomposite materials suitable for forming into macroscopic magnets, potentially enhancing the performance of traditional magnetic devices such as transformers, magnetic bearings, actuators, motors, and electrical generators. Here we discuss FeCo alloy nanoparticles, whose low coercivity and large saturation magnetization make them valuable for applications requiring magnetically soft materials. A radio frequency (RF) plasma torch is used to synthesize the particles from powdered metal starting materials and simple gases in a scalable process that is easily adapted to a wide variety of metal, carbide, and ceramic systems. Acetylene gas is injected into the plasma to coat the surface of the particles with carbon. In addition to providing an effective barrier to oxidation and corrosion [6], the carbon overcoat is expected to prevent coarsening and particle coalescence during compaction. It also modifies interparticle magnetic interactions and could reduce eddy current losses in high frequency applications. 121
Mat. Res. Soc. Symp. Proc. Vol. 501 ©1998 Materials Research Society
plasma torch
Ar + powder + acetylene
porous Metal
gas + powder
Ar plasma gas
filter
injection probe I
sheath gas
7 induction
reactor *_
coils
cyclone
(a)
2 ]reactor
(b)
Figure la. Overview of RF plasma torch synthesis showing plasma
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