Investigation On Preparation And Characterization Of Carbon-based Magnetic Composites

Due to their outstanding structure and properties, carbon-based materials have been widely applicated in the field of nanoelectronics, nanomaterials, nanomechanics and nanobiology. Especially, carbon nanotubes (CNTs) and silicon carbide (SiC) are two of carbon-based materials exhibiting excellent optical, electronical and thermal performances. In case of doping or encapsulating magnetic materials, they also possess magnetic properties, resulting in more potential applications. Therefore, research on the carbon-based magnetic composites is very important from the view point of science and technology.The objective of this dissertation is to study the special thermal and magnetic properties of carbon nanotubes and silicon carbides doped or encapsulated by magnetic ions or magnetic particles, with the following up exploration of biomedical applications of Fe₃O₄-carbon composite nanofibers in cell separation. More details are summarized below:1. Study on magnetic properties of Cr-doped 6H-SiC diluted magnetic semiconductors. 6H-SiC single crystals doped in Cr at doses of 0.0156 wt % were prepared in a stainless steel autoclave at 460℃.The doped samples were characterized with Raman spectroscopy (RS), X-ray fluorescence spectrum (XFS), Electronic Paramagnetism Resonance (EPR) and Superconducting Quantum Interference Device (SQUID) measuring their magnetic properties. Room temperature paramagnetism of the 6H-SiC single crystals due to Cr³⁺ was examined by EPR spectra, with the fine structure parameter g=1.99.To the sensitivity of SQUID analysis, signatures of ferromagnetism were observed in the samples at low temperatures, with apparent Curie temperature of 69.8 K.Before our study, it is not found any reports focused on the magnetic properties of Cr-doped 6H-SiC.2. Study on thermal oxidative properties of magnetic Fe nanoparticles encapsulated in carbon nanotubes. Thermal oxidation of Fe nano-particles in nanoscopic reactors of carbon nanotubes (CNTs) was investigated by treatment of them in air environment ranging from room temperature to 800℃.It is found that encapsulated Fe nano-particles are thermally stable up to a relatively high temperature of 170℃when comparing to bare ones getting oxidized at room temperature. Fe nano-particles in the tube end of CNTs is easily oxidized to form inverse spinel structure Fe₃O₄/γ-Fe₂O₃ nano-particles,serving as barriers to prevent oxygen diffusion into the inner hollow cavity of CNTs. It is suggested that 170℃is the critical temperature (T_c) of oxygen diffusion through the barriers. At temperatures lower than 170℃,oxygen molecules cannot diffuse through the lattice of Fe₃O₄/γ-Fe₂O₃ nano-particles to form oxygen molecules in the cavity of CNTs, while at higher temperatures the penetration of oxygen occurs and Fe nano-particles from the tip to inner most in CNTs were gradually oxidized. Due to their comparative thermal stability, Fe/CNT composites are proposed to apply their ferromagnetism in wider temperature interval.3. Study on surface functionalization for biomedical applications of Fe₃O₄-CNT composite nanofibers. Carboxylic acid groups (COOH) were introduced onto the walls or ends of the individual Fe₃O₄-CNT composite nanofiber by sonication in a mixture of sulfuric acid/nitric acid. The carboxylated products exhibit favorable magnetic properties and excellent dispersibility in water, which are excellent for use in cell separation. It is found that human ascites cells could internalize these carboxylated Fe₃O₄-CNT composite nanofibers by endocytosis, resulting in successful magnetic separation.