Microstructures and magnetic properties of carbon nanotube/Co-oxide nanocomposite powders

Abstract

Carbon nanotube embedded cobalt oxide (CNT/Co-oxide) nanocomposite powders were synthesized by molecular-level mixing followed by calcination. The surface morphology of the fabricated nanocomposite powders shows Co-oxide coated CNTs homogeneously dispersed in the Co-oxide nanopowders. XRD patterns reveal that CoO–Co₃O₄ nanopowders were obtained as a complex phase. The nanocomposite powders exhibit ferromagnetism with a saturation magnetization of 75emu g⁻¹, while CoO–Co₃O₄ nanopowders without CNTs are paramagnetic. These results indicate that incorporating CNTs into Co-oxide nanopowders triggers a change of the magnetic properties towards ferromagnetism.

Introduction

Incorporating carbon nanotubes (CNTs) into inorganic matrices such as metals and ceramics has attracted much attention in structural and multi-functional applications due to their superior mechanical, electrical and thermal properties [1]. There have been several meaningful progresses [2], [3], [4], [5], [6] in CNT-reinforced ceramic and metal matrix (CNT/ceramic, CNT/metal) composites. For example, Zhan et al. [2] found a highly enhanced toughness of Al₂O₃ by CNT addition and Peigney et al. [3] reported a novel fabrication process for CNT/Al₂O₃ nanocomposite powders with high surface area and improved mechanical properties. Cha et al. [5] showed both enhanced toughness and hardness by CNT addition, simultaneously. Till date, most of the research has been focused on improving the mechanical properties of CNT/ceramic composites [2], [3], [4], some of them are also studied for use as electronic conductors or thermoelectric materials [7]. However, there are only limited reports on the magnetic properties of CNT/ceramic composites [8], [9] because of the problem of severe agglomeration of CNTs in the inorganic matrix the problems associated with the usually employed powder metallurgy processing have not been efficiently solved, yet. However, the recently developed fabrication process of mixing metal salts and functionalized CNTs at the molecular level [10] is promising to allow for a homogeneous dispersion of CNTs in ceramic matrix materials.

Here, we report on the fabrication and the magnetic properties of CNT/Co-oxide nanocomposite powders in which CNTs surrounded by a Co-oxide shell are well mixed in nano-sized Co-oxide particles. Pure Co oxide material without CNTs was also fabricated under the same conditions such as to elucidate the effect of CNT addition. We find that the paramagnetic behavior of Cooxide materials is changed into ferromagnetic behavior by addition of CNTs. This transformation of the magnetic properties by addition of CNTs may be described by Co clustering concept and disordered spin states near the CNT/Co oxide interface.