Hydrothermal Synthesis And Characterization Of CoFe₂O₄ Nanoparticles

Cobalt ferrite(CoFe₂O₄) is a magnetic material which has good performance, such as strong magnetic anisotropy, high coercive force, moderate intensity magnetic saturation, remarkable chemical stability and mechanical strength, which has been widely used in catalyst, magnetic recording materials, iron fluids, etc.. In addition, it can be used as microwave absorbing material because of it can keep high real and imaginary parts of complex permeability in C-band and Ku-band of microwave. Nanoparticles have small size effect, surface effect, quantum size effect and the macro-quantum tunneling effect, etc., and at the same time magnetic related characteristic physical length is just in nano class(1-100nm), such as magnetic single domain size, the super-paramagnetic critical size, length of the exchange, as well as the average e-free journey, etc., when the size of magnetic materials equal to the characteristic physical length, some electromagnetic properties or physical characteristics will change. So it is very important to research the preparation methods of cobalt ferrite nanoparticle. At present, the methods that prepare nano-magnetic material mainly include mechanical grinding, self-propagating combustion synthesis, sol - gel method, co-precipitation, hydrothermal method, micro-emulsion method, etc.. Among them, the hydrothermal method become one of the most potential ways to prepare nano-magnetic materials with some advantages such as simple, low-temperature requirement, easily controll particle size and distribution of products by changing the reaction conditions, reduce or avoid reunion, high purity and needn't high temperature calcination, etc..In this paper, CoFe₂O₄ nano-powder was successfully prepared by hydrothermal method using FeCl₃·H₂O and CoCl₂·6H₂O as raw materials in second deionized water, aqueous solution of polyethylene glycol, aqueous solution of glycerin and aqueous solution of glycerol/polyethylene glycol resepctivily. X-ray diffraction (XRD), field-emission scanning electron microscopy (FESEM) and vibrating sample magnetometer (VSM) were used to study the effect of different reaction medium, pH of solution, reaction time, temperature, the ratio of glycerol and water on the structure, morphology and magenetic properties of CoFe₂O₄ nano-particles. The results showed that spinel CoFe₂O₄ nanoparticles can be prepared in the four medium conditions. when using the second deionized water and polyethylene glycol solution as medium, the as-prepared CoFe₂O₄ spherical nanoparticles are smaller and the distribution of particle size is narrow, but the crystal structure is not perfect and the Ms, Mr are lower; when using glycerol solution and glycerol / polyethylene glycol solution as medium, the as-prepared CoFe₂O₄ nanoparticle is larger, and its crystal structure is better, the Ms, Mr are higher; The growth of CoFe₂O₄, crystal structure and magnetic properties will be enhanced with the increasing of pH, reaction time, temperature and the ratio of glycerol and water; when the pH value is morer than 11, the reaction temperature above 200℃, reaction time more than 8h, the ratio of glycerin and water more than 1, most CoFe₂O₄ nanoparticles are the octahedral structure, and the crystal structure is more perfect, the distribution of particle size is narrow.