Research On The Magnetic And Magnetoviscous Properties Of Ferrofluids

In the present thesis, water based Fe3O4 and CoFe2O4 FFs as well as PAO oil based Fe3O4 and CoFe2O4 FFs with different volume fractions were prepared and characterized, respectively. Two metal-based magnetic functional fluids, Ga-Sn and Ga-In magnetic liquids, were fabricated and investigated by doping with Fe73.5Nb3Cu1Si13.5B9 amorphous particles and nanoscale Fe3O4 particles, respectively. The magnetoviscous properties of water and PAO based FFs were investigated using high-temperature rotating viscometer, MCR rheometer and torsional oscillation cup viscometer with a ring-magnetic field. A viscosity-magnetic field hysteresis effect was proposed and studied.X-ray diffraction, scanning electron microscopy and vibrating sample magnetometer methods have been used to characterize the structure, morphology and magnetic property of Fe3O4 and CoFe2O4 nanoparticles, respectively. The results show that both of the Fe3O4 and CoFe2O4 nanoparticles are inverse-spinel type structures. The particles display spherical or spherical-like shapes with typical size in the 10-30 nm. The Fe3O4 nanoparticles exhibit superparamagnetism and higher saturation magnetization than CoFe2O4 nanoparticles with low remanence and coercivity. The hydrodynamic size distribution and physical size of the FFs were characterized by dynamic light scatter and transmission electron microscope, respectively. The results indicate that the water based FFs as well as PAO based FFs have narrow size distribution with large amounts of small particles and few big particles. The physical sizes of the nanoparticles in the FFs are in the range of 13-15 nm without obvious aggregation. The magnetic relaxation of FFs were investigated by a dynomag AC susceptometer, which demonstrates that the water based and PAO based Fe3O4 FFs are Neel relaxation while the water based and PAO based CoFe2O4 FFs are Brownian relaxation. The magnetic hysteretic curves of FFs were measured by SQUID-MPMS3 magnetometer. All the FFs exhibit superparamagnetism. Our experimental data agree well with Langevin model and the results are corresponding to the results of DLS and TEM.The saturation magnetization of the amorphous particles Fe73.5Nb3Cu1Si13.5B9, is about 125 emu/g, nearly 50% larger than that of Fe3O4 crystalline particles. It is discovered that magnetic fluids, Ga85.8In14.2 and Ga91.6Sn8.4 alloy liquids, with Fe73.5Nb3Cu1Si13.5B9 particles exhibit high saturation magnetization as well as low coercivity and remanence. Furthermore, the magnetic hysteresis curves confirm that the liquid metal-based magnetic functional fluids with Fe73.5Nb3Cu1Si13.5B9 particles have higher magnetization than the metal-based Fe3O4 fluids. We found that both of Ga85.8In14.2 and Ga9i.6Sng.4 alloy based Fe73.5Nb3Cu1Si13.5B9 FFs show a temperature sensitive of magnetization. The magnetization and saturation magnetization of these FFs decrease with increasing temperature.The viscosity properties of FFs without magnetic field were investigated. The viscosity of FFs decreases with increasing temperature, due to the reduced speed difference between the carrier liquid and magnetic particles. The viscosity of FFs decreases with increasing shear rates. There is a non-linear relationship between shear stress and shear rate, which indicates that the FFs are non-Newtonian fluid. The applied magnetic field has obvious effect on the viscosity of FFs. The viscosity of FFs decreases with increasing shear rates under magnetic field, which shows shear thinning behavior. The hysteresis curves of viscosity-magnetic field show that both of the water based Fe3O4 FFs and PAO based Fe3O4 FFs exhibit different hysteresis curves with different volume fractions, while all the water based CoFe2O4 FFs and PAO based CoFe2O4 FFs demonstrate same hysteresis curves. In addition, the Fe3O4 FFs and CoFe2O4 FFs with same volume fractions have different hysteresis curves. A parameter ??*=(?de-?in)?in, is proposed as viscosity-magnetic field hysteretic effect. This parameter is named as "negative" viscosity-magnetic field hysteretic effect when ??*<0. The results of our experimental indicate that for all the CoFe2O4 FFs, the relative reduced viscosity is positive ??*>0; for the water based and PAO based FFs with 1.5% volume fraction, the ??*?0. While for the water based and PAO based FFs with 6% volume fraction, the relative reduced viscosity ??* is positive (??*>0) at high magnetic fields and negative (??*<0) at low fields. It has been found that viscosity-magnetic field hysteretic effect is affected by the magnetic force the hydrodynamic force among the particles. The formation and destruction of chain-like or drop-like structures lead to the formation of negative viscosity-magnetic field hysteretic effect in concentrated FFs.The magnetization and magnetoviscous properties of water based Fe3O4 FFs with 6% volume fraction before and after zero-field freezing were investigated. The saturation magnetization of FFs after freezing is greater than that of FFs before freezing due to the breakage of aggregations in FFs. Our experimental data agree well with the model of gas-like compression. The viscosity of FFs after freezing decreases with increasing shear rates under magnetic field, which is shear thinning behavior. The viscosity-magnetic field hysteretic curves of condensed FFs after freezing are initially studied. The results demonstrate a special curve that the relative reduced viscosity ??* is positive (??*>0) at high magnetic fields where the magnetic force plays dominant effect. The relative reduced viscosity ??* is negative (??*<0) at low fields where the hydrodynamic force plays dominant effect. The result is similar to the results of FFs before freezing. The magnetic hyperthermia effect of 6% water based Fe3O4 FFs,6% water based CoFe2O4 FFs, and 6% water based Fe73.5Nb3Cu1Si13.5B9 amorphous particle magnetorheological fluids were studied. The results indicate that the Fe3O4 FF exhibit more significant heating effect than CoFe2O4 FFs, while heating effect of the Fe73.5Nb3Cu1Si13.5B9 magnetorheological fluids is better than Fe3O4 FF because of its high magnetization.