Weakly Magnetic Field Assisted Synthesis Of Nano-particles Of Fe₃O₄ In Oxidation Co-precipitation And Properties Of The Magnetic Fluid As A Transfer Media

The morphology of nano-particles of magnetite (Fe3O4) has an important impact on the relative properties and application in engineering due to the specifications. In this dissertation, a weakly magnetic field (<500Gs) assisted oxidative co-precipitation method was proposed to synthesize the nano-particles of Fe3O4 with different morphologies. The as-synthesized nano-particles, which were modified via a surface modification method, could be dispersed in a base oil to prepare the magnetic fluids. A magnetic fluid stability testing device was manufactured to evaluate the stability of the magnetic fluids with the nano-particles using different chemicals. The rheological characteristics of the magnetic fluids with and without an external magnetic field were also investigated. The effect of the morphology of the nano-particles on the transmission properties of magnetic fluids was analyzed by a designed magnetic fluid transmission device, which was optimized via the simulation software and the numerical analysis technique.The nano-particles of Fe3O4 with different morphologies (i.e., irregular, octahedron and six-party flake) were synthesized using NaOH, FeCl2 and H2O2 as raw materials in a designed oxidation co-precipitation synthesis device with weakly magnetic field assistance. The synthesis was carried out at various reactant concentrations and magnetic inductions. The analysis by scanning electron microscopy (SEM), X-ray diffraction (XRD), fourier transform infrared spectrometer (FTIR) and vibrating sample magnetometer (VSM) shows that the morphologies of nano-particles of Fe3O4 synthesized appear irregular, octahedron and six-party flake as well. The application of weakly magnetic field to the synthesis could accelerate the phase transformation fromα-FeOOH to Fe3O4, shorten the reaction time and improve the purity of the products. The magnetic field could not have an effect on the lattice shape of the product, but could affect the morphology of the crystalline grain. The nano-particles with different morphologies possessed different magnetic properties due to the shape anisotropy and the crystalline (or particle) size. The saturation magnetization of the nano-particles with the shape of six-party flake was lower than that of the nano-particles with the shape of octahedron.In this dissertation, the stability of the magnetic fluid and the particle dispersion in the magnetic fluid were analyzed via the theoretical calculation. A test device for the magnetic fluid stability, which was designed and manufactured based on the principle of L-C oscillator circuit, was used to determine the parameters (i.e., surfactant concentration and assistant surfactant concentration) for the preparation of the magnetic fluids. The magneto-rheological properties of the magnetic fluids with the nano-particles with different morphologies were examined by a rheometer in the absence and presence of a vertical or horizontal magnetic field. The results show that the magneto-rheological properties in the absence of a magnetic field follow a Newtonian fluid behavior. The viscosity of magnetic fluid at various shear rates in the presence of a horizontal magnetic field could not be varied. The viscosity of the magnetic fluid in a vertical magnetic field increased with increasing the magnetic induction. The rheologic behaviour of the magnetic fluid became a Bingham plastic fluid in the presence of a vertical magnetic induction. The viscosity of the magnetic fluids containing the nano-particles with the octahedron shape changed with the applied magnetic field.A mathematical model for the magnetic fluid transmission was established according to the working principle of the disk-type magnetic fluid transmission device. The experimental device for the magnetic fluid transmission was designed. The materials used and the magnetic circuit in the device were determined by the simulation method and the numerical analysis technique. The models at different disk drive spaces in the device were established via the finite element method (FEM). The relation between the transmission torque and the current intensity at different disk spaces was analyzed. The main parameters (i.e., current intensity, disk space, speed difference and particle morphology) in the measurement of the magnetic fluid transfer torque were investigated in the magnetic fluid transmission device. The results show that the transfer torque was correlated to the magnetic field intensity in a certain gap between the drive plates when the magnetization of magnetic particles does not reach a saturation value. The transmission torque increased with increasing the magnetic field intensity. However, the magnetic particles gradually reached the saturation magnetization when the magnetic field intensity was further increased, leading to the constant transmission torque. The gap between the drive disks had an impact on the transmission torque. The greater gap could give the smaller transmission torque. The speed difference between the transmission disks had a little effect on the transmission torque at a lower speed. The transmission torque increased with increasing the speed difference. However, the transferred torque would decrease when the speed difference exceeded a certain limit due to the shear-thinning effect of the magnetic fluid. The morphology of the magnetic particles in the magnetic fluid could affect the transfer torque. The nano-particles of Fe3O4 with octahedral morphology could transfer a greater torque rather than the nano-particles with irregular or six-party flake-like morphologies.