Crystal Structure-controlled Synthesis And Characterizations Of Co Nanoparticles And Co-Ni Alloy Sub-micro Particles

The study on how to adjust and control the properties of materials by controlling the morphology, size and crystal structure of nano-particles have attracted much attention in the research of nano-materials, for when the size of the particles decreases to nano-scale, the morphology, size and crystal structure of nano-particles will affect the performance of materials. In this study, we used a solvent-thermal polyol process to control the crystal structure of Co nano-particles, and successfully prepared single-phase HCP or FCC and HCP and FCC phase-mixed Co nanoplatelets. We also prepared the FCC Co-Ni alloy sub-micro particles over the whole composition range and preliminary studied the mechanisms of crystal structure transformation in the preparation. The structure and the magnetic properties of Co nanoplatelets and Co-Ni alloy sub-micro particles were characterized.(1) Single-phase HCP or FCC and HCP and FCC phase-mixed Co nanoplatelets were synthesized via a simple solvent-thermal process, in which ethylene glycol was used as both solvent and reducing agent, and cetyltrimethylammonium bromide (CTAB) as control agent. It was found that the CTAB concentration in the reaction solution had a grate influence on the final structure of the products. All the samples synthesized with the CTAB concentrations between 0 and 50 mM for the reaction time of 5 h were the pure FCC phase. For the reaction time of 15 h, with the CTAB concentration in the reaction solution increasing from 0 to 5 mM, the FCC and HCP Co transformed to the thermodynamically stable HCP phase after the redox reaction. For the reaction time of 15 h and the CTAB concentrations in the reaction solution between 5 to 9 mM, the single-phase HCP Co nanoplatelets were obtained. But this phase transformation did not occur if the CTAB concentration was above 10 mM. Magnetic measurements revealed that both of HCP and FCC Co nanoplatelets were ferromagnetic. The saturation magnetizations (Ms) of the FCC and HCP Co nanoplatelets were same and 148 emu/g, smaller than the bulk value (161 emu/g). The coercivities (Hc) of the Co nanoplatelets with the FCC and HCP crystal structures were 76.9 Oe and 198.4 Oe, respectively. Compared with the FCC phase, the HCP phase showed higher coercivity. (2) Co-Ni alloy sub-micro particles having a FCC structure were prepared over the whole composition range through a simple solvent-thermal polyol process. The particle size of Co-Ni alloy was about 200 nm. XRD result revealed that the as-synthesized Co-Ni alloy was FCC structure, which did not change with the composition. Their lattice constant increased with increasing of Co content. Magnetic measurements revealed all the Co-Ni alloy particles were ferromagnetic. The saturation magnetizations (Ms) of each alloy increased with the increasing of Co content. The coercivity of the Co-Ni alloys increased with the increasing of Co content.