Synthesis and Characterization of Structure Controlled Nano-cobalt Particles
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Synthesis and Characterization of Structure Controlled Nano-cobalt Particles Shiqiang (Rob) Hui1 , Mingzhong Wu1 , Shihui Ge1 , Dajing Yan1 , Y.D. Zhang1 * , T.D. Xiao1 , M. J. Yacaman2 , M. Miki- Yoshida3 , W. A. Hines4 , and J. I. Budnick4 , 1
Inframat Corporation, Farmington, CT 06032 Department of Chemical Engineering, University of Texas, Austin, TX 78712 3 Texas Materials Institute, University of Texas at Austin, Austin, TX 78712-2201 4 Physics Department and IMS, University of Connecticut, Storrs, CT 06269 2
ABSTRACT Nanostructured cobalt particles with and without a ceramic coating have been synthesized using a wet chemical method. The structure and magnetic properties of synthesized powder were characterized using x-ray diffraction (“XRD”), high-resolution transmission electron microscopy (“HRTEM”), and a Quantum Design (SQUID) magnetometer. The cobalt nanoparticles are of either face-centered cubic (“fcc”) and/or hexagonally close-packed (“hcp”) crystalline structures. The average grain size is ~14 nm for cobalt (either fcc or hcp) with an amorphous silica coating, and the average grain size is ~9 nm for hcp cobalt and 26 nm for fcc cobalt without a silica coating. The effect of annealing temperature on grain size and magnetic properties are addressed.
INTRODUCTION Cobalt has been known with allotropic forms including fcc, hcp, epsilon (“? ”), and bodycentered cubic (“bcc”). Hull first reported the existence of fcc and hcp-cobalt after analyzing different patterns of metallic powders prepared by several methods in 1921 [1]. The ε-cobalt, a complex cubic primitive structure (P43 32), was recently recognized by Dinega et al. through a detailed structure analysis [2]. A non-equilibrium bcc structure, which does not naturally occur in bulk materials, was also obtained using epitaxial growth [3]. The fcc structure is thermodynamically preferred at higher temperatures and the hcp structure is favored at lower temperatures. The ε-cobalt can be converted to common hcp and fcc-cobalt by annealing at temperatures of 300o C and 500o C, respectively [4, 5]. However, the fcc structure appears to be stable phase even below room temperature when the particle sizes are smaller than 20nm [6]. Temperatures of about 200o C are enough to trigger atom diffusion and phase transformations for nanostructured cobalt crystals [7]. The fcc and hcp phases of cobalt are close-packed and nearly isoenergetic crystal structures that differ only in the stacking sequence of atomic planes in the [111] direction. Low activation energy for formation of stacking faults often results in the formation of both phases in the same sample under high- temperature crystallization techniques, such as melting-crystallization and evaporation-condensation. On the other hand, ε-cobalt is most often found in nanostructured particles prepared at low temperatures by solution phase
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Author to whom correspondence should be addressed; email: [email protected], Tel: 860487-3838, Fax: 860-429-5911, Inframat Corp., Willington, CT 06279
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